Thanks Jochen,

no mixups present at all, file assigned to UID 501.

Since this problem started few hours after the Debian upgrade, I think
it is related to it.

I don't know if something has changed on the NFS client side on Debian,
but I don't think so as aptlistchanges didn't notify me about it, nor if
Dovecot 2.2.17 treat NFS in other way.

I'm stuck.

On 13/07/20 16:07, Jochen Bern wrote:
> On 07/13/2020 03:45 PM, Claudio Corvino wrote:
>> in addition the "permission denied" error is
>> random, most of the time Dovecot works well.
> In *that* case, I'd say that UID/GID mapping problems can be ruled out.
>
>> How can I check the mappings NFS uses?
> You don't have any relevant options in the client's fstab entry, and
> I'll assume that there are none in the server's /etc/exports, either.
> That leaves only potential default mappings, which should be documented
> in the corresponding manpages.
>
> Also, since there's only *one* user/group involved, you can always
> "chown" a test file on one side and check with "ls -n" on the other to
> verify whether there are mixups.
>
> *Intermittent* failures of an NFS mount over a well-functioning LAN ...
> I'm thinking "file locking" now, but that's a *complicated* topic, to
> say the least ...
>
> https://en.wikipedia.org/wiki/File_locking#Problems
> https://unix.stackexchange.com/questions/553645/how-to-install-nfslock-daemon
>
> Regards,