I experience the same issue. I guess, the issue is the base URL https://kalender-bs-ddos.gmx.net/ of my calendar, which is not part of the certificate delivered. Only https://kalender.gmx.net/ and some other sub domains of gmx.net are part of this certificate.
I added my calendar providing email address. I will try to figure out, if I can re-add the calendar providing URL and email address.