Backend error after trying to buy something from traders or flea

Okay, the issue is because at some point this profile was used with a mod that adds weapons. Somewhere in your profile these modded items are still present.

When you try to buy an item the server tries to find a slot to place your new item, it does this by checking the sizes of existing items in your stash. One item in your stash is your a mod weapon. As the mod no longer exists, the server dies trying to find its size.

Solution1: Go through entire profile and remove modded items by hand

Solution2: Use mod that adds weapons, log into profile and remove modded weapons from profile, log out, remove mod

closing thread as mod related