Could be voltage drop during cranking.
Your batteries needs to be in good condition and have high CCA rating.

I have a new battery installed 2 weeks ago, with the proper rating.

And all this takes place with the engines off and only ignition on.

With the cap removed from the sensor and the ignition on, one pin on the sensor cap (not the sensor itself) should have a solid +5 Volts DC. It's the regulated DC sensor supply voltage from the computer. Another pin should be a soild connection to ground.

These could be checked with a Fluke meter.

Update of the situation,

Thanks to a stand up guy I could do some more fail search.

I have TPS low range on stbd and transducer low voltage (which is the grey reference voltage)

On strb I only have 0.36V on the grey cable that is the reference to the sensor. This goes for all the sensors. The blue cable only gives 0.24V with the throttle on idle and 0.35V on full.

I I take the sensor from port side I get the same result. => Not sensor related issue

I then swapped boxes, and the fault code follows the strb box to the port engine BUT
when I measure port sensor it shows 5V on grey and 0.5V to 4.6V on blue.
However strb grey cable voltage still remains at 0.36V

Or is the boxes programmed as port/strb? Thats why its saying port low range when its still stbd that has low voltage?

It sounds like the fault hasn't been cleared from the original stbd box and you have a wire harness or sensor cap problem with the stbd engine.

Does the strb engine still show a fault using the port box?

Stbd engine still only has 0.36V on the grey reference voltage cable.

Port engine has the right voltage on everything. But if I pull the working sensor from
port engine it says stbd engine tps fault. => The box is calibrated to stbd and port.

So boxes are ok. Why do I only get 0.36V on the grey in the whole system?

It seems The 5 volt reference voltage is not getting through your wiring harness on your stbd engine. That could be either an open connection in that line or a short circuit in another sensor pulling down that voltage (assuming it's correct comming out of the computer). Any of these problems can be tracked down using that voltmeter on the ohms function if you know how.

You are right!

I took out the grey cable from the multiconnector (disconnectsensors from the harnes sensor side)
Then I get 5V on the box side

Ive unconnected all the sensors i found but still no more voltage
in the harness. According to the scheme the grey cable splits up into
all the sensors - but where I thought that could be a good point to start from?

Or is it a better easier way to track down issue along the cable?

Disconnecting all the sensors is the right place to start. Are you sure you got them all? Seawater pump, port and stbd exhaust, crank position, IAC, manifold pressure. I can't think of any others but that 5 volts goes a lot of places. Unfortunately I don't have a schematic in front of me.

Try a resistance reading from the box end to each sensor with the ignnition off. Should be near 0 Ohms.

I got:
Seawater
Camshaft ( below the waterpump)
IAC (No grey cable here?)
MAP
TPS
ECT

Missed the exhaust sensor, also crank or is that the camshaft? Will try that tomorrow, now Ill just have to gain some knowledge in measuring, and collect a few more ideas on what to try next.

Do I have to remove the grey cable from the box to do the Ohm reading?

Thanks for the input Nighthawk, stand up guys on forums helping newbees like me is great and Im very thankful.