"/give" command won't work

[cheeserush]

Title: "/give" command won't work

Version: 1_0_3

OS: Client: Windows 7 Home Premium Service Pack 1, Server: Ubuntu 12.04

Java Version: Java 7

Description of Problem:

I'm playing on a server. When I use /give with essentials, it gives this error in game: net.minecraft.inventory.IInventory.getContents()[Lnet/minecraft/item/ItemStack

I think something's preventing plugins from accessing players' inventories. When players sell to Chestshop shops, they keep all their items but still gain Iconomy money. It's as if the inventory modifying part of the transaction failed to take place.

Error Messages:

net.minecraft.inventory.IInventory.getContents()[Lnet/minecraft/item/ItemStack

Error Log:

No error log for /give command.

(Tekkit version is 1.0.6, NOT 1.0.3)