I would like to re-start the discussion. I tried rhaferkamps patches and they are working. The original code still fails to sync with mailbox.org.
The difference is that the original code includes the hostname in the request while the patch omits the hostname.
I would like to help to get mailbox.org working with KDAV again, but I don't know how. I can't think of a solution which might work for both cases and I wouldn't like to add a server specific code to to change the URL.