YANMAR 4JH3-HTE Diesel Engine RPMs drop and then it quits

We ran under engine for 30hrs motor-sailing from Brunswick GA to Chesapeake Bay.
We turned the engine off when we got a bit of wind to sail. It was off for an hour.
Running one hour it first time suddenly reduced RPMs and died.
First thought was that we caught a fishing line. But that didn't seem to be the case.
We started the engine again and it ran fine under load for a couple of hours then it
failed again reducing RPMs. I took it out of gear, put the Morse throttle more forward, it didn't speed up and died.
We continued under sail for two more days. It failed a few seconds after starting and trying to increase RPMs w/o load except for the alternator.

I have checked so far:
Razor fuel filters are clean and no water. I have not switched to the alternate Razor filter.
The ONAN Gen-set ran fine several times for one to two hours each since the first engine failure.
It feeds from the same fuel line.
I checked the fuel filter at the YANMAR engine.
- no air bubbles when bleeding and pumping with the manual pump
- no debris and no water from the bottom drain valve
- the fuel sensor shows open for both cases, engine off, engine running and dying.
    The panel doesn't show an error light even with switch disconnected, open ended or short circuited. I don't know whether it is wired up at all or lamp is dead.
- I disconnected, cleaned contacts, and reconnected the magnetic shut off valve connector.
    My assumption is, that the magnetic valve only shuts off fuel while activated by pushing the STOP button on the panel.
- Engine temperature always was around 80°C
- Oil and cooling liquids are all looking healthy
- engine fails with and without mechanical load
- the engine is from Year 2003 and has 2750hrs
- we had the engine serviced a week before we sailed: engine oil changed, fuel filters checked, main engine belt exchanged, impeller changed.
    The mechanic didn't put the new quarter $ size rubber disc. I installed it later.
Visual inspection after we experienced failure I discovered that the starboard forward engine mounting bolt is rusty;
looking above I found a small corrosion caused water leak of the elbow water mixer for the exhaust, just below where the flange/pipe  from the turbocharger comes in.
This seems to be a few drips only and must have been going on for a while given how rusty the engine mount nut and bolt look. So I don't think this is the root cause of my problem.

We are now anchored south of Fisherman's Island at the mouth of the Chesapeake Bay near Cape Charles in a calm. Outlook for some wind tomorrow evening.
I am running out of ideas and looking for suggestions from fellow AMEL owners.

--
Michael & Robyn

SY RIPPLE SM2K # 417
currently near Cape Charles US Chesapeake Bay entrance