I have found a way to fix this without losing any data:
- Rename /Users/<username>/Library/Group Containers/UBF8T346G9.Office/ to UBF8T346G9.Office.OLD
- This backs up your existing data and settings.
- Restart Outlook. It should now start, but show no data. Don't worry -- we saved it in the last step.
- Close Outlook.
- Delete /Users/<username>/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile
- This is the empty profile that Outlook created when it started, which you will replace with your backed up copy
- Copy /Users/<username>/Library/Group Containers/UBF8T346G9.Office.OLD/Outlook/Outlook 15 Profiles/Main Profile to /Users/<username>/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/
- Restart Outlook and it will prompt you to re-authorize with your Office 365 account info before you can download new mail.
If you have multiple profiles or if it's not named "Main Profile", you might need to do a bit more work to be fully recovered, but this should get you started.
-Shane