NC 12.0.4 Updater fails

I am running 12.0.4 on a Ubuntu server.
The Updater correctly shows :"A new version is available: Nextcloud 13.0.0"
But when I click ‘Open Updater’ I get this error message:
»Could not start updater, please try the manual update
Connection to server lost«

I finally found out what the problem was.

The admin user was not in a group that was allowed to use the updater app.
Maybe a hint would be helpful

An the updater should not be accessible in this case…