Thanks for the reply.
No, I didn't hit the "take all" button, that's sure. About maybe selling the backpack, sure, that's what I thought too at first. I tried retracing the steps from the first save where it disappeared and also replaying the first save where I had it, so that I could see if I could've indeed sold/dropped it. Tried this for a while but couldn't find anything. The autosave is so old that I rather looked for it like that instead of reloading. But it's very unlikely that this could've happened, not only my bartering character only has one backpack for such things I don't usually sell backpacks, always find some use for them or leave with another character.
I will give this save game editor a try. Thanks again.
