Sync with Nextcloud, DAVx5 or direct

I'm trying to synchronise my Nextcloud contacts and calendar with my Android phone. I use DAVx5 and it works very well with the contacts app and the calendar app. However, I can't integrate it anywhere in OneCalendar because access via DAVx5 is apparently not possible. It can't be that you can't access device accounts. Is this a programme error?
Oh yes, you can specify Nextcloud under Accounts, but this did not work with all variants and settings.
Too bad, these problems/errors will probably force me to look for an alternative.

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