Dear friends, I must thank you for this precious information that I followed to the letter and the machine is on its way! It is with great emotion that I declare this thread "resolved"! Thank you again!

Hi,
Great you could solve the problem. I got a similar machine and I have exactly the same problem. Could you send me how did you solve the problem? That would be a great help and can save a lot of time.

Yes, it was easier than I feared: the clog was in the heat exchanger, like suggested by JRising.
I unscrewed the outlet of the flowmeter at the heat exchanger inlet and inserted a small copper wire. I poked at the blockage until I felt the pipe was more or less free. Then I turned the machine upside down and poured vinegar down the pipe. A few minutes later I was brewing my first vinegar-coffee.