Thanks for the precise answer! The threads in the bike are worn out. I fixed the problem with a bolt with longer threaded part, than the one that was sitting there, and a wing nut that I filed to a shape that fitted in the area where the bolt ends (the "special" nut that sat there before was made of some kind of soft [alu?] alloy). And with some help from a long bent nail that I fastened to one of the wings of the nut (of course I pulled the nail away afterwards) I got the arrangement in place! It was the best and simplest solution that I could come up with, and now it's possible to thighten the bolt and all the parts fits tight together. And it's much easier to get the new arrangement in place than it was with the special nut, which had to be held in place with a pair of tongs, and it fits better also.
/ Henrik