Weird problem with mixer's headphone out - need help

I'm currently restoring an old mixer I bought for cheap. There's this problem with the headphone output:
The signal it puts out is the master bus summed to mono with one channel phase-reversed internally. So when you dial the pan on a mono signal it gets quiet in the middle, but loud when panned to the side. The outcoming signal is also out-of-phase. Does this sound familiar to someone? I couldn't find any visible signs of fault. Everything in the mixer iis handled via 5560 opamps. Any hint where to look first? Could this be an opamp gone south?

Thanks!
Gregor