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.
Thanks for reporting this bug. The game number is AFAIK all that is necessary to diagnose gameplay related problems (like this), the server name might be important for connection issues.
I can reproduce this bug both in your game and in a test game I have set up. It seems to be only related to debt + -1coin token. Autobuy correctly figures the token into account.