Realy weird AC problem

[Jolly Roger]

I’ve got a really, really strange AC problem, which I’ve been trying to solve for a month. It’s beyond my knowledge of marine AC, so I ask for suggestions. Here’s the story.
I have two water cooled marine AC units on my 45-foot boat. One is a seven-year-old Pamponette Air 16,000 Btu, in the forward part of the boat. The other is a 16,000 Btu Marinair unit, less than a year old, mounted aft, and cooling the aft cabin and bathroom. Both units also blow into the saloon.
Water supply is common Through a seacock, filter and large 120v impeller pump. From the pump the supply divides using an adjustable Y connector to the front and back units. Each unit has its own outlet, front and rear.
So, here’s the problem: Both units work fine during the day, reducing Florida’s daytime 90/95°F temperatures to a nice 75° throughout the whole boat. However, soon after my wife and I go to bed, usually about 10.30pm the rear Marinair AC soon displays a fault code E4, and the compressor no longer comes on. The temperature soon rises to 85°+ and humidity returns. I have to switch it off, wait about ten minutes, then switch it back on, when it runs for about half an hour then conks out again E4. This repeats all night, but come daylight, about 7am, everything is back to normal and the unit runs perfectly all day.
This is not conducive to a good night’s sleep.
E4 indicates a shortage of water flow, and the manufacturers said that must be the problem, and told me how to test it. You disconnect the outlet and see how long it takes to fill a five-gallon bucket. Mine took 45 seconds, which is spot-on per specification. I have adjusted the water flow through the Y valve and increased it to the rear unit – still nothing!
I have cleaned the evaporator radiator, run it without an air filter, switched of the forward unit and deflected all the water though the rear unit, all to no avail, and now I’ve run out of ideas.
Any suggestions would be most welcome – quick please.