Something doesn't work correctly when setting up recurring events and then deleting single events in my setup. There are two different types of malbehavior I could observe:

(1) When the organizer of a recurring event series which has been existend for some time deletes one of these events, an invitee using Microsoft Outlook 365 receives a message which Outlook considers "not supported". The corresponding sanitized .ics file, taken from the invitees inbox, is attached. When the invitee opens the .ics file, the meeting is not updated correctly (i. e. in this case, it is not deleted from the invitees calendar). The behavior is reproducible for this specific series of events.

(2) When trying to provide steps to reproduce this behavior, I came across a different behavior. When setting up a new series of events and then deleting one of these events, the deletion message doesn't cause the behavior described above in Outlook 365. However, the deletion in Outlook of the deleted event doesn't take place, instead, the title of the event is permanently changed for all events (!) of this series into "The meeting "Test" on [date and time of the deleted event] has changed".

I assume that the first type of error is influenced by the fact that the series of events is older and has changed often (i. e. there already have been deletions in the past), while the second type of error happens for new events.

I am currently running SOGo 5.9.0.20231122-1on Debian Bookworm. The client software is Thunderbird, connected via CalDAV. The invitees client software is Outlook 365. I think that this is a new type of behavior, however, I changed from ActiveSync to CalDAV some time ago, so I don't know if this change is related.