kmail (in Kontact) allows no expiry of read messages

Hi y'all!

Kontact Version 5.15.3 (20.08.3)
on Debian Bullseye (sid unstable)
This is an installation which I upgraded from Debian Stable to Unstable
(Bullseye).
The button "Expire read messages after" just cannot be checked.
The button "Expire unread messages after" can be checked.
This applies to all of my folders local as well as IMAP on my local
Dovecot server.

Also I realized that Kmail lost all expiry adjustments which I had in
place before I went from Debian Stable to Testing and then to Unstable.
Everything is stuck now at no expiry (showing 30 days default) but it
can't be changed.
This is really unfortunate.

Any hints on what I might check?
Any logs or other information which might be informative?
With akonadi-console debug I got no hints about what might be missing
(file permissions etc?).

Thank you for your time
--
Eike Lantzsch ZP6CGE