My guess:
The joker valve didn't fail..it's not supposed to prevent slow seepage, which is what happened. Two possibilities: 1. The tank is full and your tank vent is blocked...which pressurized the tank. The contents had nowhere else TO go but back up the head discharge line into the toilet. 2. The vent is bloced, but the tank isn't full...AND your head discharge line has a long uphill run to the tank...in which case there could indeed be enough waste in the line that couldn't make it into the tank because of the pressure created by the blocked vent to overflow the bowl. But either way, your tank vent has to be blocked.The two most likely places for a vent blockage are the vent thru-hull and the connection--the fitting and that end of the vent line--at the tank. Scrape out the thru-hull...but before you take the vent line off the tank, open the deck pumpout fitting first to relieve any remaining pressure. Otherwise, you could get a VERY nasty bath!And don't use the toilet again or attempt to pump out until you solve the problem.