had a look in the gitlab issue: it is a nextcloud-server issue where /contact_birthdays should return a better http status or icalendar RFCs should already handle this use-case (well, davx on subscribing) but I can’t find a property for read-only vevents.
The resource is not writeable, davx wants to write, dav server claims 404, but really it is either a 403 forbidden (that davx can handle better) or some other status specific to caldav that fits better.