First of all, this isn’t a restored SFOS backup? Because those don’t work. You have to completely delete the account, then recreate it.
Are you sure you tried this:
???
If you did, and entered all the correct details (NB username is sometimes the full email address, sometimes only the part before @) and it still did not succeed - then I guess it really is time to leave GMX behind. May I recommend mailbox.org or posteo.de. Also consider getting some form of eternal email redirect. It sucks to tell your contacts to use a new addresss from now on…