Loot showing red "x0" when you pickup items from custom crates
https://streamable.com/56fb9p

it shows it in your inventory, but it's a "ghost" item, doesn't stack, can't use it, disappears when you logout, etc. if you drop it, then pick it up, it shows you picked up nothing in red bar above health/food.

In the LootTables.json file
make sure you did not set any  "Min": 0  
or "ItemsMin": 0,
This can create ghost items