Clearly I had made sure beforehand it was the relais which was faulty by reusing the same relais used elsewhere on the board. And it was faulty indeed. So I replaced it by the new Hella relais and presto it was working again. Yes, another item of the list.
I have to admit I don't like the new plastic Hella relais though. The old Bosch relais with the metal housing looks so much more nicer. Despite it's all locked away and not visible I still prefer the old ones. I have seen used ones for sale on Ebay but that does not make sense to me as chances are bigger they can go faulty soon.
Another thought that occurred to me is that the other ones might go faulty/flaky soon as well. In this case the fault was instantly noticeable (and not a big deal) but what about if for example the radiator fan(s) stops working due to a faulty relais. So I'm now thinking to get them all replaced. Would be nice to get some NOS Bosch relais switches (if anyone knows of a batch I'm interested to hear from you). Else I will buy new plastic Hella's. I will also get the fuses replaced by new ones. I discovered one broken fuse used for the heated rear window.