it seems that the debug does not work in exadata environment, does it?
it seems to work only when the TEST and DEBUG sessions are in the same instance

environment info:
PL/SQL Developer
Version 14.0.0.1961 (64 bit)
01.128889 - 1 user license
Service Contract: 01.01.2021
Windows 10 Build 18362

Physical memory : 16 654 944 kB (8 151 376 available)
Paging file : 19 145 312 kB (4 093 124 available)
Virtual memory : 137 438 953 344 kB (137 434 056 392 available)

Parameters
C:\Program Files\PLSQL Developer 14\plsqldev.exe

Preferences
Session mode: Multi
OCI Library: <none>
Use OCI7: False
Allow Multiple Connections: True

Preference Files
C:\Users\marek.abram\AppData\Roaming\PLSQL Developer 14\Preferences\default\Default.ini
C:\Users\marek.abram\AppData\Roaming\PLSQL Developer 14\Preferences\Marek.Abram\default.ini

License File
C:\ProgramData\PLSQL Developer\aalf.dat

Debug file
C:\Users\marek.abram\AppData\Roaming\PLSQL Developer 14\PlSqlDev.elf

Plug-Ins
*Active Query Builder (C:\Program Files\PLSQL Developer 14\PlugIns\ActiveQueryBuilder.dll)
*PL/SQL Documentation (plsqldoc) (C:\Program Files\PLSQL Developer 14\PlugIns\plsqldoc.dll)
(* is Active)

Aliases
ARENDUS.MTA
ASY_DB
ASY8_DB
DLC_Koidula_RDT
DLC_MAIN
...

Homes
OraClient12Home1 (C:\oracle\product\12.2.0\client)
OraClient12Home2 (C:\Tarkvara\oracle_rt\product\12.2.0\client)

DLLs
C:\Tarkvara\oracle_rt\product\12.2.0\client\bin\oci.dll

TNS File
\\failid.intra.rmv\Adminnidele\Skriptid\Kasutajatele\RMIT\oracle\tnsnames.ora

Using
Home: OraClient12Home2
DLL: C:\Tarkvara\oracle_rt\product\12.2.0\client\bin\oci.dll
OCI: version 12.1 (12.2.0.1.0)
Oracle Database 12c EE Extreme Perf Release 12.2.0.1.0

Character Sets
Character size: 3 byte(s)
CharSetID: 871
NCharSetID: 2000
Unicode Support: True
NLS_LANG: ESTONIAN_ESTONIA.UTF8
NLS_NCHAR_CHARACTERSET: AL16UTF16
NLS_CHARACTERSET: UTF8

Process
Working Set = 246 722 560
Memory = 78 556 976
GDI Objects = 1897
User Objects = 3042
Handles = 1118

Monitor
PixelsPerInch = 96

Id = 0
PPI = 96
Primary = True
Handle = 273940687
Left = 0
Top = 0
Width = 1920
Height = 1080

MainFormOnTaskbar = True

The Oracle Debug API (sys.dbms_debug) internally uses dbms_pipe messages to communicate between the controlling session and the debug session, which apparently can fail on RAC systems. The only workaround is to modify the tnsnames.ora file to connect to only 1 node of the cluster without load balancing and failover.

that was important information, thanks
Marek