VW T4 2.5 TDI shuts off / won't start

Hello, my father is having a problem with his T4, and I'm hoping someone here can help.
Is this a 2.5-liter TDI engine with 102 horsepower, engine code ACV, manufactured in June 1996?
Sometimes the car doesn't start, regardless of whether the engine is warm or cold. Then, the yellow preheating light 'flickers' dimly, but the engine turns over. If you turn the ignition on and off several times until the preheating indicator lights up properly, it will then start.
During the ride, it sometimes stalls, and the red battery warning light comes on. If the clutch isn't disengaged, it restarts after one or two seconds.
We already replaced the 'absteller' (solenoid valve), but unfortunately, it didn't work.
Who can help?

Best regards,

Hello,

Have you checked that already?


Cause:
The engine will not start if the contact pressure between terminals 15 and 50 in the ignition switch is too low, or if the connector is not properly seated.


Another possible candidate is the power supply relay.


What does the error memory say?

Okay, so the error memory is clear.