See @Gary post below for an example for the GMX.com CardDAV url. So yours should be eg:
https://carddav.gmx.net/.well-known/carddav
If that URL still doesn’t work, I would contact GMX technical support to check the URL
Some GMX users as well have had to eg: disable 2FA (two factor authentication) if you have that enabled in your GMX account. Just a thought.
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