assuming both TID 7020615 & 7012098 (no files directly under /offiles/ )

I would do a contents check with the Attachment File Check as well, might be worth to make sure that has been done for whole PO

I did it all.

I checked the directory structure.

I did almost all kinds of gwchecks.

Maybe I have to open a case - but this customer (university) does not have free (included)  cases.

sleeping on it brings a thought,   we can tell what was the last successfully archived message, so from that may well be able to determine the message or small range of messages on the mailbox in question that are triggering the error.   If not really needed, just purging it or getting Retain to bypass that small time range may be all we need to do.    

under 'About',  down at the bottom is the 'Retention Date Utility'
https://support.microfocus.com/kb/doc.php?id=7020297

Good idea, Andy.

I will create a Retain job just for this user and let start the job from the very beginning.

I had similar problem and was directed to get SOAP Trace log, in that log you can see, which message is triggering the error.