If its a Strida1 the rear pulley has 2 flanges (one each side) but the Front pulley has only one … and the belt tries to ‘run up hill’ towards it. As commented above, the belt tension is important not too much and not too little as in manual and tips.
So, if the belt is spooling off sideways on the REAR pulley is the outer flange missing ?
This outer flange is a press/bonded fit to the pulley - but if badly worn it may have worked loose - if so … epoxy or superglue back. Look after that early rear pulley as they are pretty rare now.