I didn't check the "How to Report a Bug" section until after I'd already closed the game, so I don't have the game/server number but I do have the number of the game (#3084331). On my Turn 17, I had one Debt token and the -Coin token from my opponent's Bridge Troll. The option to Pay Off Debt showed up and I clicked on it to repay the Debt. After that, my game froze but I was able to continue chatting. I noticed the Log said
I played a Copper, returned the -Coin token (which should have brought me to 0 coins), and then paid 1 Debt (with no Coins to do so). After an Undo I was able to continue the game and played all of my treasures before paying off Debt.