Enable trace if using scan listeners and verify when the problem comes as this is noticed in case when load is high
Additionally there are few similar issues reported in following Note
adop fails with "ORA-12516: TNS:listener could not find available handler with matching protocol stack" (Doc ID 2105049.1)
Lot Of Concurrent Requests Are Getting Errored REP-0501: Unable to connect to the specified database. (Doc ID 1612002.1)
Concurrent Mgr: Rep-0501: Unable to Connect to the Specified Database. (Doc ID 273681.1)
Intermittent REP-0501 AND ORA-12520 Errors When Running Concurrent Requests (Doc ID 1059717.1)
R12: CE: Autoreconciliation Errors: REP-0501: Unable to Connect to the Specified Database and ORA-12537: TNS:Connection Closed (Doc ID 2050174.1)
ADREPGEN Hangs With REP-00501 and REP-00309 (Doc ID 212806.1)
Please check whether the above Notes helps to resolve your case, else
Thank you for the reply
Yes forms working as expected when Concurrent issue occured but with little delay
I have a schedule of purge Concurrent Request and/or Manager Data Program
Can you check,
tnsping $TWO_TASK was taking longer time.
Did you find any issues in DB listener.log ?
Once bounce application and database issue resolved.
Thanks! it could be a momentary issue due to database(init.ora) or OS Level(ulimit) resource issue.