APP-FND-01564: ORACLE error 6502 in afscpcon

Problem: 
While trying to access forms through the EBS R12 login page, we received this error and for some reason I couldn't figure out for few hours what this error could be, as this environment was a fresh clone from our production environment.

Unable to login into Oracle Applications, the following error occurs.

ERROR: You have encountered an unexpected error. Please contact the System Administrator for assistance.Click here for exception details.

Exception Details.
oracle.apps.fnd.framework.OAException: Could not load application module
'oracle.apps.fnd.sso.login.server.MainLoginPageAM'.

## Detail 0 ##
oracle.apps.fnd.framework.OAException: Application: ICX, Message Name: ICX_SESSION_FAILED.

Cause:
I re-visited all the applications post clone steps and hence I found out what's the issue, as per the client's requirement we had to end date all the users except the list provided by them. Looks like one of my fellow DBA unknowingly end dated "GUEST" user.

The problem arises during the session validation after the session creation. The responsibilities for the GUEST user were disabled so that the connection could not be established.

Guest user is end dated which can be checked using the Sqlplus below:

SQL> SELECT user_name,TO_CHAR(start_date,'DD-MON-YYYY'),TO_CHAR(end_date,'DD-MON-YYYY') FROM FND_USER WHERE user_name = 'GUEST'; 

Solution:

• Guest user is then enabled via Sqlplus by setting the end_date column to null.

UPDATE fnd_user  SET end_date = null where WHERE username='GUEST';

• Login into Oracle Applications directly using Forms interfac

Go to Define user screen -> query GUEST user -> set end_date to null -> save the form -> exit

Note: Never End Date Guest User in Oracle Applications

To summarize
Oracle internally uses a login named GUEST, prior to invoking validation of actual username. Some people regard this as a security threat, but it isn’t. We as DBA’s can change the “guest” password from its default value after installation.
"GUEST" user login to connects to the database and validate with fnd_user table and authenticate your sessions, so GUEST user should ALWAYS be active.
By default, its password is oracle.

HTH
Thanks for reading..!!