Error message: Looks like something went wrong!

When you open your O365 add-in and get this error message:

Looks like something went wrong!

An error occurred authenticating your user. If the problem persists then try and reopen the add-in or contact your administrator.

This is because your email in outlook, doesn't match your email in Planner.

Please have your Planner administrator update your Planner user, so that the email matches.