I can’t reproduce the problem. I’m using DAVx⁵ + Nextcloud 15 every day and I dont have these problems, nor are there other reports about such problems at the moment.
Please provide exact steps to reproduce so that I can have a look. Without being able to reproduce a problem, I can’t say anything about it. Also, please provide Web server and CalDAV/CardDAV server logs taken while the problem occurs, DAVx⁵ debug info (DAVx⁵ / Settings / Debug info) and verbose DAVx⁵ logs etc as described in [READ BEFORE POSTING] What’s required to diagnose a problem.