FPS Submissions failing for December

Posting this in case it is of any help.

I notice a few people have been asking about FPS submission issues. I tried replying to someone else's  post submitted on Dec 30th but this forum doesn't seem to let me so I'll post this as a new entry here. Why is this forum so hard to use?!

I was unable to submit FPS for December 2025. The rejection reason was

Reason: The supplied IRmark is incorrect.
Code: 2021

The strange thing is that I'd made no changes to our payroll settings and these submissions had been successfully received previously. I did the usual things, deleted payroll and tried again without success. As the XML sent to HMRC looked identical to the previous submissions (apart from "Taxable Pay") I came to the conclusion it must be an issue on HMRC side so raised a case but was surprised that I was the only one having issues but maybe it was a "holiday period" issue.

Several weeks later HMRC have just notified me that they cannot progress the case any further as they are convinced it's an issue with the payroll software.

A bit dejected and fed up I tried once more and I have just deleted the payroll entry again and resubmitted. To my immense relief and surprise it has just been accepted.

I have no idea on which side the fault lay (HMRC or QBO) and probably never will!