"Error while preparing to send sharing message" in Outlook

article #1063, updated 1126 days ago

If you see this while trying to share a calendar to another Exchange or Exchange Online user, thus far this has been seen to be caused by (a) duplicate Permissions user entries in the calendar, and (b) user entries in the calendar belonging to deleted, disabled, or otherwise invalid user accounts. Remove the duplicates or invalids, and it goes away!