Hello everyone,
I have just been able to solve the problem by myself. In the instructions on:
https://help.univention.com/t/how-to-kopano-ucs-4-4-latest-install-dkim-support/11725
the DKIM configuration in the file “/etc/default/opendkim” is edited. This does not work in my case. However, if I copy the entries there into the file “/etc/default/opendkim”, it now works.