I got it (but not in deep detail):
I was some plugin within Nextcloud. I just logged in as admin and I updated four plugins. At least I remember the official mail module, also I had a warning on unsupportet PHP version, I think it was the password-app. Additionally, I removed some (including phonetrack) and (re-)installed some new (including phonetrack)… but sorry, I can not remember plugins I touched in detail which. After that, it was able to log in to the regular deskopt as I was used to.
But if I click the Nextcloud-Logo in the top right corner, I end up on the mobile webside again. I am not sure, but I think it is about the “radio” plugin.
TY
Thorsten