The cause for and solution to the error message "Database mount ID mismatch" in your previously configured standby database

If you have a previously configured standby database, and you have converted it to a free standing database, no longer a part of a data guard setup, you may see some errors in the alert log looking like this

2022-05-03T12:52:33.896905+02:00
RFS[1332]: Assigned to RFS process (PID:128748)
RFS[1332]: Database mount ID mismatch [0xb40d4ed8:0xb42c30e2] (3020771032:3022794978)
RFS[1332]: Not using real application clusters

Reason:
Even though you have used the data guard broker to stop log shipping, and activated the standby database (inn effect making it read writable), the broker will not stop the previously configured primary database from shipping logs to its previously configured standby destination.

Solution:
Cut off the log shipping from the previously configured primary database completely by either

1) changing the value of log_archive_dest_state_2 from enabled to defer:

alter system set log_archive_dest_state_2=defer scope=both;

or by

2) removing the value of log_archive_dest_2 altogether:

alter system set log_archive_dest_2='' scope=both;