Only when pedaling, coasting is fine. Next thing is to replace those damn pulleys and try different settings on my bearings, altough I already tried different settings before with no luck ... I always leave some play in them when off the bike.

Thanks again, man.

Next few steps I am going to take:
- pulleys replacement
- different rear bearing adjustment
- new chain again (KMC Z8S instead of shimano HG50; KMC Z8S was on this very bike until I replaced it and it worked fine, but was already stretched about 1.5%)

Another thing I probably didn't mention - I took off the chain and reused the ordinary pin, which obviously started to come out. I have replaced that pin with a reinforced one the very same day. Could the plate be destroyed? I am anyway switching back to KMC with a master link as I hate cleaning that shimano crap on the bike.