Did you try picking up barrels, etc until you are actually over the weight capacity, and then drop them, so see if that resets the status?


What is your strength? Depending on the game difficulty setting, a large strength potion boosts the stat by about 40, so there would be a large temporary increase in weight capacity. Unfortunately that does not help once it wears off.
The weight capacity is given by; Weight = 60*Strength + 800

Maybe if you try hex editing yourself a spare stat point to add to strength, that might update the overweight status? I have not checked this procedure in the after game BF, but I would expect the same pattern to hold (I can check it if you wish, or can not get it to work).
Hex editing - gold, experience, stat/skill points


Do you have a save near the end of the game? Perhaps redoing the final fight and entering the after game BF again with the patch installed will help. Maybe clean out your hero's inventory of any unneeded quest items, etc in case the problem was something in the inventory that triggered the overweight status on act change.