I would put a new 431 regulator on the B side and see what happens. Also if you set your meter to beep for continuity, check each pair of legs on each transistor to be sure that none of them are shorted out. It can be challenging (for me at least) to sort out whether the C4S is allowing the 5670 to draw too much current or if the 431 regulator has failed. Based on your voltages, a shorted 431 regulator is very plausible, and they do fail from time to time.
