@mrlpr Then I don’t have any ideas because it works here. Logs say:
2016-06-01 14:46:05 8 [HttpClient$1] <-- 401 Unauthorized https://carddav.web.de/CardDavProxy/carddav/.well-known/carddav (1768ms)
Maybe there’s some special setting like two-way auth activated for your CardDAV?