Thank you both for your replies. I've shot a roll with the same camera a couple of weeks ago without this problem, so I think the error might have happened in the loading of the tank. I loaded the film in what seemed to me complete darkness. My "darkroom" is a bathroom with a tiny window up high, that I first covered with a towel tight to the inner window frame and then I hanged a dark wool-blanket over the entire window frame. I didn't register any light at all. But could it be too bad of a solution for a darkroom? I will try to load my next roll in a room without windows.