This issue arises due to the way the lookup of other user's copies of the event is implemented. The method "_lookupEvent" from SOGoAppointmentObject.m is supposed to find the copy of a given event belonging to a given user. In order to do this, is calls the method findName on all the folders of that given user and takes the first object it finds. However, the findName method doesn't always return nil in case nothing was found, but sometimes may also create new objects. More specifically, the findName method from SOGoGCSFolder.m will instantiate new objects if the currently processed request is an HTTP "PUT" operation. This also happens with attendee status updates via CalDAV, as these are announced to the server using the "PUT" command. In this case, the status update may erroneously proceed with this new object instead of searching the other calendars. Depending on which folder is searched first, the actual object may be found and the update sometimes works as expected.

The attached patch fixes the problem by rejecting new Objects from lower layers in the _lookupEvent method. However, my gut feeling is that the proper fix would actually consist of changing findName to purely lookup things instead of mingling it with object creation.