Where is my problem? "Uploading of the element "XXX" failed"

Hello,

I hope I am right here.
I have connected my GMX account with the em-Clienet and also use the calendar actively. Whenever I send myself an appointment with my business e-mail address and accept it, the error “Upload of element XXX failed” appears after synchronization.

Maybe someone can help me on how to fix this error?

PS: The problem only arises when I accept invitations from other people.Appointments created by me do not generate any errors.

best regards

Nick