[pnb]

Could be a blocked solenoid downstream. That message most likely coming from the spinning water meter close to the tank, but possibly also the tank float/sensor. So a blockage downstream or a weak pump could mean the water meter not spinning as no flow: hence message. Those meters easy to open: rotate top about 40 degrees. Careful not to snap pipes off. Some gunk in there could be preventing rotation with low volume flow (for steam)

You getting good flow from hot water is confusing as it uses the same circuit path as steam from what I can see. Only difference is with steam it puts small amounts of water into the flash boiler to generate the steam as opposed to continuous higher flow for hot water. Solenoids probably a good bet if water meter looks ok, but do a descale first if you have not already done that.

Other things you can try if you can figure out for your model is resetting the calibration (forget the term) (this is not a factory reset) and try getting into test mode and activating the solenoids manually to see if they sound consistent (all sound different) as the coils can get weak with time leading to premature closing or failing to open. Hack for that is to convert from AC to DC but easier just to replace.

 

[pnb]

Could be. Sometimes you can swap solenoid positions in these as a test: depends on the circuit design and if you have multiple 2 or 3 ways.
Another thing you can try if you can make the error happen on demand is to disconnect tube from the flash boiler to solenoid and see if water/steam at that point when error produced.

 

[pnb]

Another thing to check..make sure pipes before pump are staying full of water. If a tiny pinhole, you could get air in them which could prevent water meter turning when low flow. Cockroaches love chewing in the new pipe compound. Also try to determine whether water meter rotation signal is consistent, and obv free rotation.

 

[pnb]

If you can figure out how to get into the service menu on your model (combination of dregs draw and button presses most likely) you might find something like this: "encoder reset"
this basically gets the machine to learn the pulses from the various sensors from scratch I believe and have used it to correct an oversized puck when the machine could not relearn.
Certainly worth trying if you have no luck elsewhere.

 

[pnb]

Air in the system will definitely cause that, but one would think making a few coffees and running hot water would flush that out.. unless between tank and pump... either hose loop or in the water meter.. I normally manually prime that circuit after taking them off.

 

[pnb]

Well, I think a solenoid stays open while steam is being built up and it goes to the drip tray until it decides it is ready to divert to the steam wand.
But I think your issue is going to be with that connection. Could be a flaw in either the pipe surface (in the photo the OS surface looks very gnarled), mating surface (but you replaced?) or the o-ring. Personally I would just replace the o-ring and be done with it. Get a slightly oversize diameter and that soft red silicone. quite a variety of sizes in the smaller range, but you may have to put it on the pipe and grease it prior to insertion.

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[pnb]

Possibly on the sensor. but can probably test by removing and measuring ohms whilst heating.
Ditto for the thermal cut-off fuses.

But I would think... if overheating it would be showing up on both high steam pressure when steaming and more evidently: lots of steam being produced along with the hot water when doing hot water.

I thought you already replaced that flashboiler, but maybe confused with other. If those black end connectors that easy to replace, I would try that first. Can you inspect closely with a loupe?
I presume the leak coming from the end and not the side indicating a hairline fracture?
Consider oring with same external diam, smaller internal diam and a slightly thicker cross section by 0.1 or 0.2mm and installing with silicone grease.
Delonghi have that listed as 3.8mm internal and 2mm cs. So 7.8 external. From memory I thought they were smaller than that?
If that size is correct, online next size up for red is 2.5 so white silicone only other choice at 2.2mm and softer than NBR. Not sure about how those stand up to heat/pressure. Ought to be ok and if you try that then the 8mm OD with 2.2mm CS has 3.6mm ID. could be tricky to get in but ought to be possible.

 

[pnb]

Measure the actual o-ring you have prior to deciding. If you cannot get it in... I have resized an oversize oring slightly by mounting the oring on a dremel tool with similar diameter and running the OS edge against fine grained sandpaper.