thanks for the swift reply.
it’s probably not the right username (email vs user name?)/app password (extra space in copy/paste? revoked by mistake?), and/or there is a server error.
I’m sure all the credentials were correct since I did a direct copy-and-paste from the web-interface and also double checked them.
Especially confusing was the fact that when swapping the app password for the regular one made all the difference and the account was created as expected.
It all came down to the app specific password not being accepted.
you can also install the Nextcloud app and then use Login flow to generate/use the app password automatically. Does that work?
Well, I did that and it worked out fine.
I removed the account in the Nextcloud app and also revoked access for it in the web interface but kept the DAVx5 account.
This still works fine.
Here where it gets weird: trying to create an account as before, i. e. using an app password, now also works.
I cannot say what “blockage” using the Nextcloud Login flow freed up but everything seems to be in working order now.
I’ll keep an eye on it, though.
Thank you for you assistance.