Unable to sign in - check your internet connection and try again

Can't play MCD anymore - game won't even get to the main menu.

It just gives the message "Unable to sign in. Check your internet connection and try again."; see attachment.

Well I checked my internet connection - it was still there. I tried again but got the same message.

I tried a different internet connection.

I repaired the game. I uninstalled and reinstalled.

I flushed the DNS resolver cache.

Thing is, the game was working fine up until I last played it, about a week or so ago. I have changed nothing. This problem has been going on for about 3 days now.

Every other game I own is working fine. My internet connection is working fine.

"Check your internet connection and try again" is a bit vague, really. There's nothing wrong with it.

Game is unplayable.