I never had a Kopano subscription nor had I any special Kopano Webapp plugins installed.
In the meanwhile I restored a backup of the member server and so went back to 4.4-9. Then I repeated the update and the problem was the same.
I then had a look at /etc/apt/sources.list.d/20_ucs-online-component.list and indeed… on the Backup DC this file was empty (despite the comments) and on the Master DC these entries could be found (here the WebApp is installed, too):
deb https://appcenter.software-univention.de/univention-repository/5.0/maintained/component/ kopano-webapp_20220622120718/all/
deb https://appcenter.software-univention.de/univention-repository/5.0/maintained/component/ kopano-webapp_20220622120718/amd64/
I simply copied these to the Backup DC and (re-)installed the WebApp by apt-get. After that I did a “univention-app install …” Now things seem to work again.
So IMHO somhow during / after the update there must be a problem related to the template file “/etc/univention/templates/files/etc/apt/sources.list.d/20_ucs-online-component.list” which not correctly determines the existence of the webapp installation.