ORA-00479: RVWR process terminated with error followed by ORA-479

 Problem:

On a 19c DB, when trying to enable the FLASHBACK mode on a standby DB, I get the following error:

 Checking the Alertlog:

ORA-00479: RVWR process terminated with error
ORA-479 signalled during: alter database flashback on...

 Analysis:

Flashback was turned ON, on this standby DB few days back but due to a heavy activity that was happening on the Primary DB and in order to avoid having the FLASHBACK logs to fill the Flash Recovery Area, I thought to turn Flashback feature OFF and turn it ON after the heavy activity.
I've to highlight that within the time window of that heavy activity I've added many datafiles on the primary DB which I think it's the main reason behind that error.

Solution:

Restart the DB instance, then enable the FLASHBACK feature.
Unfortunately, this solution involves a downtime on the Standby DB if it's being accessed by the applications.