Perhaps you can enable Oracle Net tracing by setting "trace_level_client = USER" in the sqlnet.ora configuration file? The trace file may shed some light on this.
  1. Edit D:\app\suddray\product\11.1.0\client_1\network\admin\sqlnet.ora using notepad or wordpad
  2. Change trace_level_client=OFF to trace_level_client=USER (or add this line)
  3. Save the sqlnet.ora file
  4. Run PL/SQL Developer and reproduce the problem
  5. Tracing file is located in PL/SQL Developer directory (sqlnet.log). Please send this file to me.
  6. Change trace_level_client=USER to trace_level_client=OFF
  7. Save the sqlnet.ora file

Marco Kalter
Allround Automations