Thank you Craig.
Still I keep wondering: our Outlook client is still the same so that didn't change. In Totara 13 (our current production server) it's working fine.
But in Totara 18 (Test) it's not working as expected.
Then the only conclusion I can make is that the format / content of the iCal message is different in Version 18 compared to the 13 version. Is that correct and can developers tweak that format again?