MySQL Error wth Relay logs - Could not open relay logs file for logging
Raised the issue to Oracle Support. 

The issue is a bug where Oracle development team is still working on it.

Issue: MySQL Enterprise Backup tries to backup all the binlogs and during that time, internal mechanism cant take backup of binlogs and crashes the DB. Oracle development is still working on the issue.

Workaround: 
Avoid taking backup of binlogs and relaylogs in both incremental and full backup.