I would guess that it needs the stereo setup routine run. You also should re-calibrate it after doing any major change with the optocouplers. But this symptom could be caused by ambient light getting into one of the optocouplers. Just make sure there are no holes on the ends of either optocouplers that light can get in. If you aren’t sure, you might want to seal them (just to be safe) with something opaque (caulk, sticky-tack, opaque hot glue, etc…).
Good luck.
-Brach