For others reading this thread, the issue here was that the Posting Journal data file and the company data got out of synch with expected sequences, and the posting journal had data when none was expected. Some problem prevented the journal to be purged when it should have been.

The solution to disable the Allow Posting Journal Purge option meant that running Year End deletes the posting journal data files for you - meaning that the out of synch scenario is ignored. This was the correct procedure to follow when this problem occurs.