Update: The sign-in and activation issues have now been resolved.
If you downloaded and installed Office for Mac from your UTmail+ account, you may experience some issues signing in that are currently under investigation.
Microsoft has posted the following:
We're investigating an issue in which users may be unable to sign-in to Office 365 and are receiving an error related to Office 365 subscription activation. More details can be found in the admin portal under MO146611.