Got the backup of clients to work!
The documentation did throw me off a bit. The Bareos repository does not work for some reason, I keep getting invalid key when downloading and installing Release.key.
I added the Univention Bareos repository to the additional UCS servers to be backed up and was now able to install the Bareos-filedaemon. Subsequent configuration worked as it should.
Still to be figured out is how to install the WebUI since that is not included in the Univention-Bareos repository.
The Univention-Bareos version is 13.2.2 and I can see that version 15.2 is published.
When will 15.2 be available from the Univention AppStore