Problems with uploading confirmations to calendar invitations

When I send invitations to a calendar event and a receive a confirmation for this event, the eMClient cannot upload this information to my calendar. Instead, I receive an error message saying that “the upload of this event has failed” FOR EVERY CONFIRMED EVENT. I can choose to ignore this problem for all events, but I receive the same error messages EVERYTIME I START THE eMClient! The only way to get rid of this error is by deleting the event, which of course is not a useful option.

I use Mailbox.org as my online service to save and sync events (no problems in general).
The confirmation mails are sent by MS Outlook.
I use the latest version of the eMClient 7.2.35595.0.

If this problem persists, I cannot do anything but changing to another program.
Any solution?! I hope this problem gets fixed!