Solaris

How many groups are the user(s) that have the problems in?

Linux NFSv2 used to have a non-compliant implementation where group entries would be truncated for users in more than 16 groups. And I see non-zero NFSv2 stats in what you've posted. Seems a bit of a stretch, but are you certain your file system mounts are ALWAYS done with "vers=3". I remember a subcontractor who ignored direction from the customer and prime contractor to always use that mount option because their engineers were "smarter than the 'idiots' who told them that". Until one day the production system they delivered hiccuped during NFS negotiations and fell back to v2. On a system where many of the data files were MUCH larger than 2 gb....

When's the last time your NFS server was rebooted?

The user is only in one group.

Re. "ALWAYS done with vers=3": no, I cannot confirm that. Judging by the nfs flags from nfsstat, it claims to be vers=3, but I did not add this in vfstab.

I will go ahead and add the v=3 flag in vfstab for all of the nfs mount points and observe and report back in a while once I've thoroughly tested. Thank you for this advice already.

Re. "last time rebooted": 400+ days ago at the time of writing the original post. I have rebooted today.

What do the permissions look like when viewed on the NFS server?

A reply for posterity and momentary closure:

[Not in response to the problem stated in my original post] I ended up rebuilding the NAS (serving via NFS) after replacing all the disks and upgrading firmware. I don't know what un/related problems this may have solved.

I added the NFS v3 param to vfstab. Nevertheless, I continue to see a few client NFS v2 calls via 'nfsstat'. They are only a few and I wonder whether they originate from polling-like calls, i.e. "send a few v2, v3, v4 calls, see which ones work". Just a guess.

I have not yet re-encountered the problem, but will report back if I do. Thanks for all the suggestions for debugging.