Comment # 44 on bug 1209457 from St���phane DESNEUX

QNAP NAS is up to date: model TS-412 version '4.3.3.2211 Build 20221124'

This doesn't solve the issue.

(In reply to Neil Brown from comment #40)
> There is a known problem with QNAP which is triggered by a recent change in
> Linux NFS.  We do have the fix to avoid triggering this bug but I don't
> think that update has been released yet.  Once you get a kernel that was
> released after 22/Mar/2023 QNAP should be fine.

kernel .55 has been released after 22/Mar/2023 as reported by uname:

# uname -a
Linux <hostname> 5.14.21-150400.24.55-default #1 SMP PREEMPT_DYNAMIC Mon Mar 27
15:25:48 UTC 2023 (cc75cf8) x86_64 x86_64 x86_64 GNU/Linux

Is it the correct release date? If so, maybe the fix occurred later? I'll try
to dig in https://github.com/SUSE/kernel/tree/SLE15-SP4 ...

As mentioned here https://forum.qnap.com/viewtopic.php?f=35&t=165385 , a
workaround is to force the protocol version to 4:

# mount -t nfs nas:/Web /mnt/tmp/                 => FAILS
# mount -o nfsvers=4 -t nfs nas:/Web /mnt/tmp/    => OK
# mount -o nfsvers=4.0 -t nfs nas:/Web /mnt/tmp/  => OK

HTH