MetropleX
Thanks MetropleX for the quick reply.
I do not use a google account for the SSO. Nextcloud is hosted in a webspace. I have created a user there where I manage calendar, addresses and notes. The user is name@adress.com.
It will not fail because of a donation for the developer. I'll gladly pay it for the tool if that's the real reason (I'll also gladly pay it if it's not necessary :-)).
But since it works on my current device and I'm sure I haven't donated yet, I doubt that's the real reason. If there are any further hints in the community, I would be very grateful.