I would take your multimeter and check continuity through the cable, but, measure on the board at J1. All the signals at J2 are tied to the same signals at J1. This will give you a better idea of the integrity of J2.
You may also want to just rework the J2 end of the cable again. Clip off a 1/2 inch and start again. After all the gyration, there might be stress damage to the wires. Be sure to strain relief the J2 end when you think it is good to go.
BTW, I think you should also follow TomB's advice and cross post a link to this thread in the main grubDAC thread. It would be nice to get cobaltmute's take on the situation as he is the designer and grubDAC expert. He will certainly be able to take you fully through the design and inner workings far better than I could ever attempt to do.