Minecraft Version: 1.12.2
Inventory Tweaks Version: 1.63
Forge Version: 14.23.4.2705
SpongeForge Server Version: 1.12.2-2705-7.1.0-BETA-3361

This issue only occurs with the mod installed both client-side and server-side.

When opening a GUI which you would not typically be able to remove items from or add items to (such as a Sponge plugin's chest-style menu), this mod allows you to freely space-click and alt-click. The items are moved or dropped as they normally would be if you had a chest open.

Steps to reproduce:

• Install the above versions of everything.
• Install CrazyFeet or some other plugin with chest menus.
• Install LuckPerms on the server and give yourself the "crazyfeet" permission node.
• Run any command that opens a chest menu. (/crazymenu in the case of CrazyFeet.)
• Space-click or alt-click the items out.

I can second this happening, Huskycrates Cratekeys mod installed and VirtualChest plugins affected by this bug. Both Crates and VirtualChests restore their inventory after the player uses the "Take all" shortcut, meaning they can do this multiple times and gather infinite items.

Delete the inventorytweaks mod from the server files and it fixes the problem.

Are you using Spongeforge?

It appears he also has SpongeForge installed, since he's using those plugins.