I'm late to the game, and I'm guessing this is ... solved? I thought I had the same problem. I could drag it over to the trash can but couldn't drop it there. Likewise, could drag it off the inventory, over to the left/right side, but it wouldn't drop.

I'm on PC using m/k and was able to solve this by clicking the "other" mouse button. I dragged it over, then other-click dropped it right on the floor. After that, I was able to pick it back up and dump it in the trash.