Outlook calendar does not sync

Hi Martin, my replies: 

When she tries to change or re-save an appointment, however, she gets an error message.

--What was the error? it wasn't posted.

--Possibly from the event changing, this can break a few things, who was invited, who can change the event, cancel, etc. 

What can I do? There were many appointments scheduled for next year, and I don’t want to recreate everything from scratch.

--This might be the solution unfortunately. Create new events, maybe try scheduling through Teams channel specific for these 4 folks, then it's web-based in a Teams account (test this first before moving ahead with all your past events). 

--Moving ahead: I was thinking about offering you a how to back up your calendar just in case something happens (still a good idea to do with this type of activity of 4 people working one calendar), restoring a back up can still not offer the perfection of editing past events though. 

Lisa