That's causing the phone's Lch hot to touch the amp's r channel+ and the phone's Rch + to touch the amps ground. Since the phones are common grounded through the cable, you've got the both channels in a phasey mono connection. aR+...pL+....p-....pR+...a-. I really doubt it's the jack or a part problem. My guess would be that it's more likely a board or solder issue.
That may be but 3 of us have the problems. We have made sure that none of the solders are touching the other and what not.