Further, in replying to invitations also appears to be left to the server (tested with both web client and CalDAV client) and therefore Kube should probably not submit a reply via IMAP too

I have tested Apple Calendar again with both Fastmail and Kolab Now accounts

Apple's calendaring apps leave this functionality to the CalDAV server. This means that an invitation gets sent out by the server (tested with Fastmail and Kolab) but I don't believe it's placed in the sent items folder as the mail app and Calendar app are separate from each other.

Of course, and as you have a workable solution it's not of critical importance now.

Why by its name @cmollekopf?

@mbohlender I've taken the liberty of adding you here. I appreciate you're pretty busy.