[Engine] 2.0 HDI. engine won't start

[oleolegua]

Hello, I have a 2002 Berlingo Mk1, 2.0 HDI, 90 hp. The car has run for over 300,000 km without any problems and now it's starting to act up: The engine won't start. The battery is new and charged, and the tank is full. The control unit was replaced with a used one that was coded by a specialist. After that, the car ran without any problems for three days (in sunny weather). Then it stood in the rain for two days and the engine wouldn't start. 

And now comes the strange thing: When I unplug the connector from the second injector, the engine starts, but of course it only runs on three cylinders and doesn't go above 2000 rpm. What could be the problem? Could it be related to the moisture? The OBD displays a few error codes, but they were already there when the engine was running. Where else can I look? I'm grateful for any advice. 

Here are the error codes my obd-reader found: P1641; P1197; P1356; P1357; P0339; P1354

[evdama]

Voltage drop from an ageing battery and/or faulty injector?
Not certain but the ecu provides the ground for the injector and it could be detecting a dodgy one