What kind of meter are you using to read the voltage? A 9V AC RMS voltage would have a peak of 12.7V, so a reading of 11.6V might not be out of the question depending on your multimeter. It seems like the power supply producing an over-voltage would be somewhat unlikely. It's an actual transformer, not a switching power supply, so if the secondary side is high, it would mean that that the primary side would be high. Slight over-voltages aren't uncommon in the winter, but one close to 30% would be unheard of.

If I had to bet, I would suspect something like a loose ribbon connector in the M13 itself.