Some time ago I faced some session on wait event DFS Lock Handle during a RMAN Backup on Standby Database. Btw, running backup on a Standby is a very interesting approach to avoid running backups on primary, so all nodes can be fully dedicated to application ends.
Turn that in my situation I noticed there was a lock with my apply process. The fix? Quite simple: Cancel apply process, run backup, and restart apply again. In my case, using DG Broker:
dgmgrl connect / show configuration show database edit database set state = 'apply-off';
— Run Backup
edit database set state = 'apply-on';
Hope it helps!