As soon as I logged in and clicked Multiplayer on the Start Screen, I was 'awarded' an empty golden chest. There was a membership item, however. I heard on the chat this has happened to someone else, in different circumstances though - it happened after a battle, with Festering Guts killing both players, making it kind of a draw. Should it happen again I'll be sure to take a screenshot.
I have received a similar bug recently. The previous day I had ended a game by killing an enemy with Festering Gut (while each of us were one victory point away from winning), and the game bugged causing us to both get a "Defeat" message followed in my case by a victory chest that was empty when I opened it. When I logged in today, it said I had a battle in progress and asked to reload. When I did it tried to load the battle from yesterday but got stuck loading it.
Be interesting to know if the OP's bug was similar in origin, because I've heard what you're describing several times before, actually [though thankfully never experienced it myself]. So if it's all one bug, that simplifies things. At least, presuming @Jon can track it down
