Just an FYI update - This issue kept recurring until I did an emergency downgrade to 5.8 (from 6.0). This was about two months ago at the end of July. Since the downgrade I've had no lookupdotdot failure messages on the file server, and given the previous pattern I believe it would have shown up numerous times by now if the problem was present in 5.8.
Unfortunately I'm not enough of a kernel / file system hacker to start bisecting this, and I'm not even sure how to go about replicating it outside of production... But it does seem to be related to something that was introduced between 5.8 and 6.0. -- A Dog
