Have you verified that you have configured the proper TNSNAMES.ORA and LISTENER.ORA file. The TNSNAMES.ORA file that Oracle Forms uses is not in the same directory as the server's LISTENER.ORA file. Oracle Forms is installed in a different ORACLE_HOME directory than the Oracle database software. So make sure that you are modifying the correct config files. Additionally, the TNS_ADMIN environment variable can point to virtually any directory; I would use Windows Explorer to perform a search for "tnsnames.ora" files on your disk drive. You might want to check to see if someone has set the TNS_ADMIN environment variable and then look in that directory for your config files.
Ensure that the TNS alias that you are trying to use has correctly defined the host, port, and database SID to match what is defined in your LISTENER.ORA. If you need to, you can post both configuration files here and I'll be happy to take a look at them.
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