As I said, my console is getting spammed with things like: "[10:18:47 INFO]: Client> [10:18:47] [Client thread/ERROR]: Item entity 32747 has no item?!".

The game will start lagging and stop responding every few seconds (or if I open my inventory), and will eventually crash. This happened in multiplayer too and crashed the whole server for some reason. The server had plugins, which I thought were the problem, but when I tested the snapshot, it still kept happening! I don't know if it helps, but I'll put screenshots of the console in here anyway.