Sorry for "digging the topic up", but maybe it will help someone...
The bug is related to changing from CraftBukkit engine to Vanilla engine. Tekkit Lite is Vanilla engine, so it doesn't support CanPickUpLoot value that CraftBukkit sets to 1, so using NBTEdit You have to modify Your players files ([world name]/players folder) changing CanPickUpLoot from 1 to 0 in every of the players files.
So simply, You can fix it by two ways. 1, simple one but clears items of Your players: Just delete all files from players folder. 2, simple too but boring way that won't delete Your players items: Download NBTEdit (search in Google, You'll find) and in every file in the players folder modify CanPickUpLoot value to 0.
Ah, and i'm sorry for bad English, i am from Poland :-)