I'm feeding mostly vendace (muikku in Finnish) which should be a safe fish, and some calsium/vitamin supplement with it, a couple of times I've given him salmon. Temps are about 23°C in the cool side and there's a heating pad under a hide (under the glass ofc) and in the hide the temperature is about 34°C.

He's captive bred, born in 2009.

I haven't seen this behavior before. Can't at least see any mites on the snake or in the enclosure. No injuries as far as I know and I just checked that there seems to be no signs of injuries either that I could see.

I don't have a humidity meter atm, but he's never had trouble shedding - and he had just shed the same day - so it shouldn't be too dry. I don't think it would be too humid either, because in the air here is pretty dry especially in the winter.