I have an issue with office 365. Today, I had 2 users with the "Couldn't Verify Account" drop-down bar at the top of their screen. I have seen this before and simply closed all office windows and then went into the windows credential manager and removed all the Microsoft office entries then re-opened and entered their credentials. This would typically solve it and did for the first user I got with.
The second user is being difficult and the above process doesn't work. I had also done this, rebooted then attempted to re-enter credentials with no luck.