The D50 outputs jacks have a switch built in, so that if just one jack is used, it receives both L and R signals. When plugs are inserted into both jacks, these switches preclude the signals from summing, so that the R output goes only to the R jack, and L to the L jack. If you are losing signal in the right jack with two plugs inserted, then I think the R signal is not there (and with only one jack plugged in, you are always hearing just the L signal, regardless of which output jack you are plugged into).
So try using headphones, and see if you are hearing both sides of the output there. If you do hear both sides, then the problem is isolated to the jack board or the wiring connector to it. That should be an easy fix.
If one side is missing in the headphones, then I would take a look at IC6 on the bender board (a 4570 op amp) and see if you are getting the right output at pin 1. For comparison, the left output will be at pin 7.