CalDAV not syncing invitations from Outlook

Hello,
we are currently evaluating eM Client as our new groupware, as our organization changed from Exchange to IMAP/SOGo with CalDAV and CardDAV. Everything works pretty well out-of-the-box and we really start to like working with eM Client.
But I still have one major issue, which is, that eM Client is not syncing all of the appointments with the online CalDAV-Calendar that are sent to me from others (presumably the problematic ones are from Outlook-users). That means that some invitations (presumably not sent from Outlook) are processed correctly after accepting, are apperaing in the calendar of eM Client and then are synced to the CalDAV online calendar. Others (the problematic ones, presumably coming from Outlook) appear in the calendar of eM Client but are never synced to the CalDAV cloud. If I choose to repair the CalDAV-sync in eM Client, they also disappear in eM Client.

I am a little lost here - any help is appreciated!
Thanks and bests
Mat

We still could not solve the problem, but it is not related to eM Client but to our Server…

Sorry, for bothering…