I had a similar problem too on my surpresso A53009 (which is quite similar inside). to find ''my'' leak I had to disassemble the machine, to eventually find out the cause of the leak was accessible without such surgery. it came from the fact the expansion box (in step 7 of the guide) had piled up so much powder that it clogged, causing an O-ring on the valve just before it to leak due to overpressure.
(However, upon reassembling everything I do get the blinks from bad contacts. Haven’t managed to get rid of it yet)