Problem:
A user receives a notice that a change has been made that requires them to restart Outlook even though they have not been migrated to Exchange Online or were previously migrated.
Resolution:
This will generally happen because the user is connected to a shared mailbox or calendar that was recently migrated. Outlook recognizes this and prompts for the restart so it can process the change. This is expected behavior.
To resolve this, close Outlook. If you have Jabber open, close it as well. Wait one minute and then restart Outlook and Jabber.