I remembered a YouTube video where a guy said that in old devices a certain type of glue would degrade and become ever so slightly conductive. Just enough to cause sensitive circuits to malfunction. And what have we got here? A big splash of that brownish gunk right over the pins of the protection IC! They had glued the patch wire to the board with it and splashed glue all over the place.
I cleaned it with acetone and the amp came back to life. I resoldered everything just to be safe that it wasn't a dodgy solder joint.
As a precaution I also took care of the other glue spots: