Calendars on remote nextcloud server are not showing

I have an account on a remote nextcloud server. I also have nexcloud and davx apps installed on my phone. Originally this all worked but very recently I upgraded the phone’s OS and now, although I can see all my calendars listed in davx they will not appear in my phone’s calendar.

Can anyone suggest a course of action please?

I would recommend to deinstall DAVx5 to get all calendar fragments removed from your phone and reinstall it afterwards. By doing so, I would expect that all calendars are recreated and visible again.