Test the DAV Groupware resource against some real DAV server running locally. Sabre seems to be a viable and fairly popular implementation.

Honestly, one of the most popular DAV servers is actually NextCloud, so it might be worth looking into running NextCloud in a container to test against, or (even better) figure out if its possible to run their DAV server standalone (which I doubt, I guess it's some PHP library wired into NextCloud core).