Have you tested with the two calendars that I posted above that were causing the issue?
These were added to my calendars in nextcloud, then tried to subscribe to them in DAVx5 but was told that I didn’t have a webcal-capable app to subscribe to them. When looking at the properties in DAVx5, it showed URLs like this https://serverurl/remote.php/dav/calendars/username/calendarname.
I changed both calendars in the nextcloud account to use http:// instead of webcal:// and they both work now, I added UK_Holidays again as webcal:// to test for the debug info, but it still doesn’t work.
If you can’t debug it from this, I might be able to create an account for you, but I’m not willing to share the details publicly, it would have to be through PM.