I can't be really specific because there are so many Eurocombis (17 !!), but you have to look carefully at the flow route through the boiler, and where the pressures for the primary flow switch are sensed. There has to be enough flow past those points for the diaphragm to move the pin.
It's "flow" x "flow resistance" = "pressure difference" ( like amps x ohms = volts)
So if the pump is working properly, either the resistance between the sensing points is too LOW (unlikely), or the flow around the whole circuit is too slow because of resistance there being too high.
Ariston are always quick to accuse dirt in the primary side of the secondary h/e , but in the one I took apart last any increase in restriction there INcreased the pressure diff at the flow switch. The problem was that the HW loop resistance was much less than the CH loop, so the pin didn't move enough on HW. SOmeone had swapped a washer on the 2ndry h/e for one with a smaller hole in the middle, which helped, but was only a botch - because the pump was knackd.
As I said it does depend where the pressure is sampled, but if the 2ndry h/e is clean, the only thing to slow the flow is the various waterways - as Weargas suggested, or the main h/e.
SO - assuming you have the book of words or enough patience to work it all out from the castings, you could have a busy weekend!
By the way was it a greeny-goldy Wilo pump with the long shaft as I mentioned?
Good luck - it might be just a flake of scale somewhere critical.