To obtain some more diagnostic information, can you modify the PL/SQL Developer shortcut and add the DebugSQL parameter? For example:

"C:\Program Files\PLSQL Developer 12\plsqldev.exe" DebugSQL

Reproduce the problem and send me the debug.txt file that is generated in the %APPDATA%\PLSQL Developer 12 directory (e.g. C:\Users\<User>\AppData\Roaming\PLSQL Developer 12).