After testing/placing a few games, I have run across this problem problem as well. The session expires because of a timeout. I thought it was just user error when others were reporting it, but it just happened to me a couple times, too. Anyone else?

I can confirm that this happens to me too.

I wish this would happen less often, it is frustrating when it happens in middle game.

"Sorry, your session has expired and the game has finished.

Press your browser's Refresh button to start again."

then the game is over.