Dumping the archived log:
For example, based on the event_log, or debug logs, the might have been some sort of issue detected on seqno=x offset=x. You can try dumping the archived log some many blocks before and some blocks after the offset to try to find out what might have happened around this time.
seqno=75222 offset=8385536. I will try to dump 1000 blocks before (min) & 1000 block (max) after the issue to see if I can find anything, by dividing the offset by your block size of 512.
sql> alter system dump logfile '<archived log destination path>/<archivedlog75222>'
rba min 75222 . 14378
rba max 75222 . 17378
NOTE: Replace the <archived log destination path>/<archivedlog75222> with your archived log destination and archived log name.