Solved it.
The reason why no update on dom0 was possible is too low memory in the update qube.
On my system the update qube is sys-firewall. I could’n increase the memory from sys-firewall because the option in qube settings was greyed out.
sys-firewall depends on a template debian-11-dvm which is a template for disposables, this in turn depends on the basic debian-11 template.
At this basic underlying template it was possible to increase the system storage max size by a few gigabytes.
This did the job, the installation of dom0 update worked.
Thanks at all for replies.