I have used eM Client with GMX for years. Since they integrated the CalDAV and CardDAV services in the automatic setup, I have never had an issue with my GMX Calendar.
I think that may be the difference. Though they are all the same company, including Mail.com, they are all configured differently.
Did you try an contact their Support? I found that after I got through the automated response messages they send, GMX.com were actually quite helpful with any issues I had.