So I have seen this twice now since the last patch, not before ...

I was inside a lobby, the host started the game but I was chatting, and I was hitting the backspace button on the loading screen that shows up for less than a second for regular players. There was a cancel button, maybe that activated it somehow?

Either way the result was that I was left staring at a blank screen of the background image, no black loading screen nothing, ESC didn't get me back to menu, nothing worked.