Despite of the problem with the app management (After new installation no app management) I achieved to activate the calendar and contacts app with the command line tool but the contacts app does not work. in the contact settings I can see two addressbooks but the app says there are no contacts and there are no addressbooks visible which I can select. Also the webdav synchronisation with the addressbooks of my computer do not work.
When I look in the forum it seems that the version 25.0.1 is very buggy.
I played around with addressbooks and there is a strange behaviour. I tried to syncronize an addressbook with one of my computer with webdav but no addresse are transfered to nextcloud. When I write an email on my computer, the used address is transfered in the nextcloud addressbook but only this one. Very strange!