Would check this note:
enable configuration; fails with Error: ORA-16721, ORA-16607 [ID 1321405.1]
I have checked the link which you have posted, but i can enable my configuration without any errors or warnings and the sync is working also so no issue with the configuration itself.
Only when i enable "fast_start failover" and start the observer this error occur up only on the primary not on the standby.
i guess i have to open a call at oracle itself.