Hi,
I also have this issue now (in version 15.0.3) -- not sure if it ever worked in version 15, I had no issues with version 14 or below.
And when right-clicking on a object for compare, nothing happens...
Logging
EurekaLog 7.10.3.0 update 3 950BEB23D70E284583494E6502C08651 AB20FBA6C9F94040B46F768C898ACD39 DA39A3EE5E6B4B0D3255BFEF95601890AFD80709 9B88E7152ED88A4DB49B528CAD729760
Application:
----------------------------------------------------------------------------------------------------------
  1.1 Start Date      : Thu, 9 Mar 2023 10:47:27 +0100
  1.2 Name/Description: plsqldev.exe - (PL/SQL Developer 64 bit)
  1.3 Version Number  : 15.0.3.2058
  1.4 Parameters      : prefpath="C:\Users\ID010709\OneDrive - TMNL\PlSqlDev\Preferences" nologon nosplash
  1.5 Compilation Date: Thu, 15 Dec 2022 15:04:16 +0100
  1.6 Up Time         : 2 minute(s), 26 second(s)
Exception:
--------------------------------------------------------------
  2.1 Date          : Thu, 9 Mar 2023 10:49:54 +0100
  2.2 Address       : 000000000084F05F
  2.3 Module Name   : plsqldev.exe - (PL/SQL Developer 64 bit)
  2.4 Module Version: 15.0.3.2058
  2.5 Type          : ETreeViewError
  2.6 Message       : Invalid index.
  2.7 ID            : E0C58323
  2.8 Count         : 1
  2.9 Status        : New
  2.10 Note         :
  2.11 Sent         : 0
Active Controls:
--------------------------------------------------------------------------------------------
  4.1 Form Class   : TPLSQLDevForm
  4.2 Form Text    : bill_owner@bons - PL/SQL Developer - Compare User Objects of BILL_OWNER
  4.3 Control Class: TButton
  4.4 Control Text : T&arget Session...
Computer:
-------------------------------------------------------------------------------------------------------------------------
  5.1 Name            : PWNVDIDEV0872
  5.2 Total Memory    : 34358714368 (32,00 Gb)
  5.3 Free Memory     : 26480869376 (24,66 Gb)
  5.4 Total Disk      : 214296424448 (199,58 Gb)
  5.5 Free Disk       : 124306255872 (115,77 Gb)
  5.6 System Up Time  : 1 day(s), 9 hour(s), 35 minute(s), 16 second(s) / 1 day(s), 9 hour(s), 35 minute(s), 23 second(s)
  5.7 Processor       : Intel(R) Xeon(R) Gold 6240R CPU @ 2.40GHz
  5.8 Display Mode    : 2805 x 1368, 32 bit
  5.9 Display DPI     : 96
  5.10 Video Card     : VMware SVGA 3D (driver 8.17.3.5 - RAM 8388608)
  5.11 Printer        : Microsoft Print To PDF (driver 10.0.19041.2604)
  5.12 Virtual Machine: VMWare ESX
Operating System:
-------------------------------------------------------------
  6.1 Type                : Microsoft Windows 10 (64 bit)
  6.2 Build #             : 2009 (10.0.19044.2604)
  6.3 Update              : November 2021 Update [21H2] #2604
  6.4 Non-Unicode Language: English (0409)
  6.5 Charset/ACP         : 0/1252
  6.6 Install Language    : English (0409)
  6.7 UI Language         : English (0409)
  6.8 Edition             : Enterprise
Custom Information:
--------------------------------------------------------------------------------------------------------------------------
  9.1 PL/SQL Developer: Version 15.0.3.2058  (64 bit)
                        01.142274 - 50 user license
                        Service Contract:_01-05-2023
                        Style:_Sky
                        Windows 10 Build 19044
                        en-US(1033)/English (United States)
  9.2 Parameters      : C:\Program Files\PLSQL Developer 15\plsqldev.exe
                        prefpath=C:\Users\ID010709\OneDrive - TMNL\PlSqlDev\Preferences
                        nologon
                        nosplash
  9.3 Preferences     : Session mode:_Multi
                        OCI Library:_C:\Oracle\instantclient_21_3\oci.dll
                        Use OCI7:_False
                        Allow Multiple Connections:_False
  9.4 Preference Files: C:\Users\ID010709\AppData\Roaming\PLSQL Developer 15\Preferences\default\Default.ini
                        C:\Users\ID010709\OneDrive - TMNL\PlSqlDev\Preferences\ID010709\default.ini
  9.5 License File    : C:\Users\ID010709\AppData\Roaming\PLSQL Developer 15\aalf.dat
  9.6 Debug file      : C:\Users\ID010709\AppData\Roaming\PLSQL Developer 15\PlSqlDev.elf
  9.7 Plug-Ins        : *Active Query Builder (1.5.0) (C:\Program Files\PLSQL Developer 15\PlugIns\ActiveQueryBuilder.dll)
                        *PL/SQL Documentation (1.4.0) (C:\Program Files\PLSQL Developer 15\PlugIns\plsqldoc.dll)
                        (* is Active)
  9.8 Aliases         : B2MA
                        B2MP
                        TAXDEVA
                        TEST
  9.9 Homes           :
  9.10 DLLs           : c:\oracle\instantclient_21_3\oci.dll
  9.11 TNS File       : c:\oracle\instantclient_21_3\Network\Admin\tnsnames.ora
  9.12 Using          : Home:
                        DLL:_C:\Oracle\instantclient_21_3\oci.dll
                        OCI:_version 12.1  (21.3.0.0.0)
                        Oracle Database 19c Enterprise Edition Release 19.0.0.0.0
  9.13 Character Sets : Character size:_1 byte(s)
                        CharSetID:_31
                        NCharSetID:_2000
                        Unicode Support:_True
                        NLS_LANG:
----------------------------------------------------------                        NLS_NCHAR_CHARACTERSET:_AL16UTF16
                        NLS_CHARACTERSET:_WE8ISO8859P1
  9.14 Process        : Working Set = 184.807.424
                        Memory = 30.935.216
                        GDI Objects = 692
                        User Objects = 788
                        Handles = 789
  9.15 Monitor        : PixelsPerInch = 96
  9.16 Windows        : *Compare User Objects of BILL_OWNER
Call Stack Information:
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Methods |Details|Stack           |Address         |Module      |Offset          |Source                 |Unit               |Class                      |Procedure/Method                   |Line                               |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|*Exception Thread: ID=12992; Parent=0; Priority=0                                                                                                                                                                               |
|Class=; Name=MAIN                                                                                                                                                                                                               |
|DeadLock=0; Wait Chain=                                                                                                                                                                                                         |
|Comment=                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|7FFFFFFE|04     |0000000000000000|000000000084F05F|plsqldev.exe|000000000044F05F|Vcl.ComCtrls.pas       |Vcl.ComCtrls       |                           |TreeViewError                      |8788[1]                            |
|00000040|04     |000000000014E8D8|00000000008522E8|plsqldev.exe|00000000004522E8|Vcl.ComCtrls.pas       |Vcl.ComCtrls       |TTreeNodes                 |GetNodeFromIndex                   |10382[27]                          |
|00000040|04     |000000000014E948|0000000001D90620|plsqldev.exe|0000000001990620|ConnectionLogonUnit.pas|ConnectionLogonUnit|                           |DoConnectionLogon                  |128[24]                            |
|00000040|04     |000000000014E9C8|0000000003E98360|plsqldev.exe|0000000003A98360|CompareUserUnit.pas    |CompareUserUnit    |TCompareUserForm           |TargetBtnClick                     |743[23]                            |
|00000040|03     |000000000014EB38|00000000007C4FC1|plsqldev.exe|00000000003C4FC1|Vcl.Controls.pas       |Vcl.Controls       |TControl                   |Click                              |                                   |
|00000040|03     |000000000014EB78|00000000007FEB06|plsqldev.exe|00000000003FEB06|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TCustomButton              |Click                              |                                   |
|00000040|03     |000000000014EBA8|0000000000800392|plsqldev.exe|0000000000400392|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TCustomButton              |CNCommand                          |                                   |
|00000040|03     |000000000014EBE8|0000000000411C32|plsqldev.exe|0000000000011C32|System.pas             |System             |TObject                    |Dispatch                           |                                   |
|00000040|03     |000000000014EC48|00000000007C48BD|plsqldev.exe|00000000003C48BD|Vcl.Controls.pas       |Vcl.Controls       |TControl                   |WndProc                            |                                   |
|00000040|03     |000000000014EDB8|00000000007CBB1B|plsqldev.exe|00000000003CBB1B|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |WndProc                            |                                   |
|00000040|03     |000000000014EE98|00000000007FE4F0|plsqldev.exe|00000000003FE4F0|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TButtonControl             |WndProc                            |                                   |
|00000040|03     |000000000014EED8|00000000007C43B0|plsqldev.exe|00000000003C43B0|Vcl.Controls.pas       |Vcl.Controls       |TControl                   |Perform                            |                                   |
|00000040|03     |000000000014EF38|00000000007CBD19|plsqldev.exe|00000000003CBD19|Vcl.Controls.pas       |Vcl.Controls       |                           |DoControlMsg                       |                                   |
|00000040|03     |000000000014EF78|00000000007CD0E3|plsqldev.exe|00000000003CD0E3|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |WMCommand                          |                                   |
|00000040|03     |000000000014EFB8|0000000000411C32|plsqldev.exe|0000000000011C32|System.pas             |System             |TObject                    |Dispatch                           |                                   |
|00000040|03     |000000000014F018|00000000007C48BD|plsqldev.exe|00000000003C48BD|Vcl.Controls.pas       |Vcl.Controls       |TControl                   |WndProc                            |                                   |
|00000040|03     |000000000014F188|00000000007CBB1B|plsqldev.exe|00000000003CBB1B|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |WndProc                            |                                   |
|00000040|03     |000000000014F268|00000000007CAC9A|plsqldev.exe|00000000003CAC9A|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |MainWndProc                        |                                   |
|00000040|03     |000000000014F2B8|00000000005D4454|plsqldev.exe|00000000001D4454|System.Classes.pas     |System.Classes     |                           |StdWndProc                         |                                   |
|00000040|03     |000000000014F308|00007FFAD9C8E7E2|user32.dll  |000000000000E7E2|USER32.dll             |USER32             |                           | (possible CallWindowProcW+1010)   |                                   |
|00000040|03     |000000000014F498|00007FFAD9C8E367|user32.dll  |000000000000E367|USER32.dll             |USER32             |                           | (possible DispatchMessageW+919)   |                                   |
|00000040|03     |000000000014F4F8|00007FFAD9CA0C1D|user32.dll  |0000000000020C1D|USER32.dll             |USER32             |                           | (possible SendMessageTimeoutW+317)|                                   |
|00000040|03     |000000000014F558|00007FFADB670EEF|ntdll.dll   |00000000000A0EEF|ntdll.dll              |ntdll              |                           |KiUserCallbackDispatcher           |                                   |
|00000040|03     |000000000014F5E0|00007FFAD8FC1124|win32u.dll  |0000000000001124|win32u.dll             |win32u             |                           |NtUserMessageCall                  |                                   |
|00000040|03     |000000000014F5E8|00007FFAD9C8DEBD|user32.dll  |000000000000DEBD|USER32.dll             |USER32             |                           | (possible SendMessageW+2429)      |                                   |
|00000040|03     |000000000014F688|00007FFAD9C8D615|user32.dll  |000000000000D615|USER32.dll             |USER32             |                           |SendMessageW                       |                                   |
|00000040|03     |000000000014F6D8|000000000080FDFF|plsqldev.exe|000000000040FDFF|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TButtonStyleHook           |DoClick                            |                                   |
|00000040|03     |000000000014F738|00000000008102D6|plsqldev.exe|00000000004102D6|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TButtonStyleHook           |WMLButtonUp                        |                                   |
|00000040|03     |000000000014F778|0000000000411C32|plsqldev.exe|0000000000011C32|System.pas             |System             |TObject                    |Dispatch                           |                                   |
|00000040|03     |000000000014F7D8|00000000008FA3E3|plsqldev.exe|00000000004FA3E3|Vcl.Themes.pas         |Vcl.Themes         |TStyleHook                 |WndProc                            |                                   |
|00000040|03     |000000000014F868|00000000008FA774|plsqldev.exe|00000000004FA774|Vcl.Themes.pas         |Vcl.Themes         |TMouseTrackControlStyleHook|WndProc                            |                                   |
|00000040|03     |000000000014F898|000000000080FEF2|plsqldev.exe|000000000040FEF2|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TButtonStyleHook           |WndProc                            |                                   |
|00000040|03     |000000000014F8D8|00000000008F9709|plsqldev.exe|00000000004F9709|Vcl.Themes.pas         |Vcl.Themes         |TStyleHook                 |HandleMessage                      |                                   |
|00000040|03     |000000000014F918|0000000000AFFF8E|plsqldev.exe|00000000006FFF8E|Vcl.Styles.pas         |Vcl.Styles         |TStyleEngine               |HandleMessage                      |                                   |
|00000040|03     |000000000014F968|00000000008F520C|plsqldev.exe|00000000004F520C|Vcl.Themes.pas         |Vcl.Themes         |TStyleManager              |HandleMessage                      |                                   |
|00000040|03     |000000000014F9C8|00000000007C91E4|plsqldev.exe|00000000003C91E4|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |DoHandleStyleMessage               |                                   |
|00000040|03     |000000000014F9F8|00000000007CB254|plsqldev.exe|00000000003CB254|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |WndProc                            |                                   |
|00000040|03     |000000000014FAD8|00000000007FE4F0|plsqldev.exe|00000000003FE4F0|Vcl.StdCtrls.pas       |Vcl.StdCtrls       |TButtonControl             |WndProc                            |                                   |
|00000040|03     |000000000014FB18|00000000007CAC9A|plsqldev.exe|00000000003CAC9A|Vcl.Controls.pas       |Vcl.Controls       |TWinControl                |MainWndProc                        |                                   |
|00000040|03     |000000000014FB68|00000000005D4454|plsqldev.exe|00000000001D4454|System.Classes.pas     |System.Classes     |                           |StdWndProc                         |                                   |
|00000040|03     |000000000014FBB8|00007FFAD9C8E7E2|user32.dll  |000000000000E7E2|USER32.dll             |USER32             |                           | (possible CallWindowProcW+1010)   |                                   |
|00000040|03     |000000000014FD48|00007FFAD9C8E224|user32.dll  |000000000000E224|USER32.dll             |USER32             |                           | (possible DispatchMessageW+596)   |                                   |
|00000040|03     |000000000014FDC8|00000000009849CE|plsqldev.exe|00000000005849CE|Vcl.Forms.pas          |Vcl.Forms          |TApplication               |ProcessMessage                     |                                   |
|00000040|03     |000000000014FE48|0000000000984A43|plsqldev.exe|0000000000584A43|Vcl.Forms.pas          |Vcl.Forms          |TApplication               |HandleMessage                      |                                   |
|00000040|03     |000000000014FEB8|0000000000984E91|plsqldev.exe|0000000000584E91|Vcl.Forms.pas          |Vcl.Forms          |TApplication               |Run                                |                                   |
|00000040|04     |000000000014FEF8|0000000003FCD301|plsqldev.exe|0000000003BCD301|PlSqlDev.dpr           |PlSqlDev           |                           |initialization                     |197[11]                            |
|7FFF7FFE|03     |000000000014FF28|00007FFAD962760E|kernel32.dll|000000000001760E|KERNEL32.DLL           |KERNEL32           |                           |BaseThreadInitThunk                |                                   |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Modules Information:
--------------------
Processes Information:
----------------------
Assembler Information:
----------------------
Registers:
----------