There's a bunch of clientside stuff being loaded including CPlayerTryUseItemPacket and Minecraft that happens when the player rightclick/shifts a mending item.
2019-09-07-3.log

Does the same error occur when WorldEdit is not installed?

Huh. Just took a look at your code, that's really odd. WE is probably messing with something? I have no idea, your code is completely serverside. welp...

Nevermind, you're using Network.sendToServer on the server! Duh! Don't do that, lol