- either use "Login with email address", or
- CalDAV URL: https://caldav.gmx.net
For CardDAV, use this URL: https://carddav.gmx.net
User name: your GMX email address
Password: your GMX password
No CardDAV contact group support. Choose "per-contact categories" as contact group method to minimize problems.
Two separate DAVdroid accounts for CalDAV and CardDAV are required because of missing SRV records.