Yes, the relay clicks, and I do measure 238V between N and O on the PM1. The LED indicating "ON" of the MRC-16 is also active. The problem is that there is no voltage on the LOAD connection of the MRC-16 where the heating cable is connected. So while writing this I halted for 5 minutes and went back down to check if the relay of the MRC-16 was broken..............
Back again! Of course, the behaviour is caused by the MRC-16! I didn't use it for heating this winter due to the electricity prices, so when I heard the relay of MRC-16 and the LED powering up I just didn't think the fault was in the original system. But the relay of the MRC-16 must be the guilty even though it clicks (it does not hold the contact closed).
Too long time doing administrative office work and forgetting my actual engineering background is my only excuse, and now I am pretty embarrassed .