I had some issue with a ASM disks. This disk was used for my FRA. Diskgroup was not in recoverable state, so I created a new DG & changed the DB_RECOVERY_FILE_DEST parameter
SQL> startup mount SQL> alter database flashback off / SQL> alter system set DB_RECOVERY_FILE_DEST='+FRA2' /
But when I tried enabling flashback again, got following error
alter database flashback on * ERROR at line 1: ORA-00600: internal error code, arguments: [krf_enable_flashback_db_5], , , , , , , , , , , 
After checking flashback related views, I found an entry in v$restore_point view.
As all of my flashback logs were gone, this restore point was not of much use. So dropped the restore point
SQL> drop restore point rp1 /
Now it allowed me to enable flashback.