debian 11, vtapes not found correctly anymore

At two amanda-installations on Debian11 I see this lately:

both setups use an aggregate setup of multiple external usb disks with 
vtapes on them.

That worked well for years.

Now I get errors because amanda does not find valid tapes on one or more 
external disks.

"amtape config inventory" shows the vtapes, but "amcheck -t config" 
tells me "No acceptable volumes found".

If I relabel a tape "amlabel -f vtape_usb vtape_usb-002-004  slot 1:4" 
it gets detected again:

$ amcheck -t vtape_usb
Amanda Tape Server Host Check
-----------------------------
mount: /mnt/externaldisk01: can't find 
UUID=98cb03d6-e95e-462d-a823-6011b37c9f42.
slot 1:4: volume 'vtape_usb-002-004'
Will write to volume 'vtape_usb-002-004' in slot 1:4.
NOTE: skipping tape-writable test
Server check took 6.673 seconds
(brought to you by Amanda 3.5.1)


(externaldisk01 is absent: OK, externaldisk02 is connected)

I assume I should grep through some debug logs. What and where to look for?

regards, Stefan