After restarting all services (Admin server, Node manager, Managed server and OPMN), users were not bale to login to Presentation Services (commonly known as answers/analysis). It was showing invalid user/password error.
Log file: /u01/APP/OBIEE/instances/instance1/diagnostics/logs/OracleBIServerComponent/coreapplication_obis1/nqserver.log
error: [2017-08-02T21:53:52.000+05:30] [OracleBIServerComponent] [ERROR:]1]   [ecid: 0000LqZjaJG4qmmpsoL6iX1PWVhs000002,0:26:1:6] [tid: eaed6700] oracle.webservices.provider.ProviderException: java.lang.RuntimeException: oracle.bi.security.service.SecurityServiceException: SecurityService::authenticateAccess is denied for the specified credentials/identity.
[2017-08-02T21:53:52.000+05:30] [OracleBIServerComponent] [ERROR:1]   [ecid: 0000LqZjaJG4qmmpsoL6iX1PWVhs000002,0:26:1:6] [tid: eaed6700] [nQSError: 43126] Authentication failed: invalid user/password.
Root Cause: The BISystemUser account has become locked.
- Log in to the WebLogic Server Administration Console.
- Navigate to Security Realms > myrealm > Users and Groups > Users > BISystemUser > Password
- Reset the password to same as of old password (i.e. you just need to reset to same password) and save.
- Login to Enterprise Manager.
- Navigate to WebLogic Domain and right click on bifoundation_domain.
- Click on Security > Credentials
- click on oracle.bi.system and then system.user
- Then click on edit on top of this table and reset the password same as you set for BISystemUser in WebLogic Server Administration Console.
- Restart OPMN services and wait (i did so) for 10 minutes before login into Answers.
- It should work.