I opened a door and started walking slower instantly. It turned out there was a red Stele with the effect that heroes have 50 percent less movement speed in dark rooms. But, for me that effect wasn't only active on dark rooms, but in every room. 

Entering a dark room and re-entering an active room did nothing. 

My co-op friend had normal, unchanged movement speed. 

Spending Dust on the Stele removed the effect like normal.

I played as Comrade.