Hello. Thanks for your awesome app!
mailbox.org exposes it’s calendars via CalDAV. There is an extra calendar (Tasks/“Aufgaben”) which which is not seen by davdroid. It has a special tab in their web ui but its caldav url is similar to the other calendars: https://dav.mailbox.org/caldav/XXX, where XXX is calendar ID. For the tasks it seems to be ID == 27, and for others its a dynamic ID.
I can import the tasks calendar fine into thunderbird and it works like normal calendars. I wonder why davdroid doesn’t see it. There isn’t a way to manually add it either.