Evidently one situation on where the ORA-19643 can happen is where an rman catalog is shared between primary and standby and then for example an INCREMENTAL LEVEL 1 is created at PRIMARY, and the STANDBY has not yet applied logs – which can advance standby datafile’s checkpoint beyond the last incremental backup’s checkpoint scn.

Thus the error ORA-19643 may be signalled intermittently because of a timing issue, i.e., it depends on when the incremental backup is created at primary and the delay in applying the logs at standby.

Is it possible that this could be the situation here (i.e. are you interleaving incrementals between primary and standby, with standby logs being applied after some (intentional or otherwise) delay?

(for ref Bug 16622161 : ORA-19643: DATAFILE 11: INCREMENTAL-START SCN IS TOO RECENT)