Ok, all systems are now on 4.2.1 errata 133.
The problem is the same as before.
The Setup is:
One Master Server with Kopano installed (existed before).
One Member Server (new) with Nextcloud and Collabora. When I selected the Nextcloud App in the AppCenter it told me, that I should install univention-ldap-overlay-memberof before. Ok, so I thought that I installed the wrong UCS server type for that purpose. However, I installed the LDAP package first - as suggested - and then the App and it worked.
Then I installed Collabora, added the UCS cert and the Collabora App in Nextcloud.
When updating the member server, it told me, that a LDAP server on this machine is not supported. So I removed it:
Then the update worked and the whole Nextcloud / Collabora setup still works as well, but there's still the error message when clicking "Save".