I appreciate very much all your hints and recommendations!
Few answers to some of the questions above:
in the diesel? I would tend to exclude that. The diesel comes from the same tank as for the boat engine (Volvo Penta) and the engine has 2 Racor
filters. I didn't see any water in them and the engine runs smoothly.
2) The generator
starts properly and runs fine on idle. Then it dies immediately (no shuttering or anything) as soon as I apply a load to it (I tried with the water heater and/or the battery
charger). It is like when you switch it off from the panel with the OFF button, but in this case it is an unwanted stop.
3) Waiting few minutes after it has died, taking the loads off, then restarting it is no problem. It starts as before, then dies again once load is applied.
4) I didn't hear any rev increase as the load is applied. Dies immediately. Either has no time to throttle up because it switches off or cannot throttle up so it switches off.
5) I looked through the manual and it doesn't mention this case. I read however that the spindle on the actuator requires maintenance
and lubrication as it may jam.
6) As far as I know the generator has been maintained properly by the previous owners and it ran just fine this summer while cruising. It just reached 1000 hours.
Anyone knows if on the Fischer-Panda there could be some kind of sensor on the actuator that drives the throttle on the injection pump, so that if it doesn't turn as expected to increase the revs once under load it will shut down the whole system?
Unfortunately, as I wrote above, I am far from the boat at the moment but plan to do some troubleshooting, follow your advise, as soon as I will be back onboard.
Many thanks again to you all!