Hey, it's a mistake server settings.
Make sure that you have the same Item.txt on the server side and on the side Item.bmd game client.
Alternatively, check the registration of all subjects in ItemTooltip.bmd
I think your Item.txt have some new items that have value 1 on drop that doesn't exist on your item.bmd that's why it drops unknown(invisible) items.
Put '//' at the front of the items that you haven't used on your item.txt and this problem will be fixed.
if i was you i would do this.
1. go to monster.txt and see what Level is the monster the +1 item drop from
lets say its level 10000
2. go to item.txt and find Items that have level 1000 (same as monster where the +1 item dropped)
2a. be sure you find the specific Item cus monsters drop random items and u might get confused, so be sure u find the actual item that drops bugged.
3. u can either remove the item then or fix it.