This "bug" happens to me when I am logged on to WGT at one computer, forget to close the window, and then log on on another computer.
You can be logged into the game client on two different machines. If you are, it seems that when you get an invite, and accept, you will start your game as normal. But as far as the other machine is concerned you haven't accepted the invite... so it sits there until you time out... and you get this error.
I don't know if this could be the situation causing the problem for the OP, but may shed some light on what is happening.