Unable to connect with Ionos exchange server anymore

Hi there, since a few weeks I can't sync anymore with my exchange server at Ionos hosting. I tried to remove the account in the OneCalendar app and even deleted and reinstalled the app.

Now I get the following error message when I try to connect to the exchange server:

"Error while adding your Exchange account: net_http_ssl_connection-failed".

I also tried to use OneCalendar without validation of the certificate with the same result.

I bought several premium versions of OneCalendar for my family to sync on all devices. Would be great if this works again ASAP.