The 50 terminal is the wire that activates the starter. The act of starting puts the car into different mapping for fuel control.
I have never seen this code, but would imagine from the description that tern 28 on the KE controller will think it is seeing the start signal. Note: the start signal is not just the battery voltage that occurs at this point during starting. It actually is the lack of ground. Notice that the 50 terminal (28 at N3) goes to the windings of the starter solenoid. When there isn't battery voltage (as in starting) there is ground through the windings. Thus if this wire is not connected the controller also sees the same thing as starting; the lack of ground.
Check this circuit. I would also advise looking at the EHA current as this will really tell you what the electronics are doing.
Have you tried the impulse codes at the other diagnostic connector by the battery? These will be more specify as to the fault.
Bosch Master, ASE Master, L1
33 years MB technician