a day of break from the belt, zinc oxide cream and baby powder. Right now my Dad bends the waistband a bit to make it fit me better to not let it happen again. Mom checked how it fits me and what may have caused the chafing in the first place. Maybe it is better afterwards.
Edit: it seems to fit me much better. The pressure of the waistband is more evenly spread over a bigger area than before.
The bit of bad news is that I feel that it's a bit more tight in the crotch area.