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The flood sensor cannot reasonably be the source of the problem. As stated in my question, the flood basin was completely dry, the microswitch operated as it should, and neither opening or shorting the connector going from the flood sensor made any difference. The drain pump operates as it should. All chambers underneath the tub seem to be properly plugged, but I do not know where the pressure chamber is located or what it looks like. However, it could not reasonably be any over-pressure either, since the condition comes on as soon as I apply power to the dw., before any washing program has started. I have left the dw disconnected from mains for 3 hours, but the fault comes back when I plug it again. It could be a stuck over-pressure sensor, how do I locate it and diagnose it? It could also be a fault in the control electronics, so my last resort would be to replace it.
I hadn't removed the panel on that side, but see it now. It certainly does look as if there is some deposits in there, although they are at the bottom in my case and not at the top as in your picture. There are two thin blue wires coming from the top, looks like some sensor (pressure microswitch?), I removed the connector, applied power and voilà, the fault did not appear. So it looks as though you have pinpointed the problem. What next: Can I clean the pressure chamber, or should it be replaced?
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Ok, anyway I'll have to remove it, so I will have a go at trying to clean it when it is out. If unsuccesful then I'll buy a new one. Thanks for good assistance.
I removed the pressure chamber, cleaned it and put it back, but as it turned out the pressure chamber was not the culprit. The dishwasher is equipped with an "Aqua-stop" system, with a magnetic pressure valve at the kitchen sink end of the inlet water tube. After some further investigation I suspected this part, took it out and checked it by applying 220V. No reaction, so I though I'd have to buy a new one, but decided to test it a last time, whereupon it reacted to power. So it was just the valve that had got stuck, and beating a little it with a screwdriver would probably had done the trick, saving me the full disassembly. The user manual could have been more explicit. Anyway cleaning the pressure chamber probably was not a bad idea in any case.
And by the way there is a "Reset error condition" procedure, press and hold "Normal 50o" and "Normal 60o" while turning the dishwasher on.
In the end your help was more of a moral nature than factual, but thanks anyway.