Using older versions of Outlook for Mac for personal email and calendar
The problem is related to an appointment the user was invited to and is in his/her calendar.
It might be also related to renaming some items inside the recurring calendar group. At some point it disconnects from the recurrency-group. But I wasn’t able to figure out how to reproduce the error.
If you are able to identify which calendar entry it is and delete it, then outlook shouldn't crash anymore.