Alright so basically i figured out why that error happens for multiplayer servers.

so basically you want to remove his actual addon which uses sbox_bhop etc that is what spits out errors constantly. You only want to use his sandbox gamemode patch which unfixes bhopping.
To do this you need to replace your player_sandbox.lua file with the one he provides in your sandbox gamemode.

After you do that i recommend using this bhop addon https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=239725852 which just goes into your addons folder.

DO NOT use his addon until he fixes it only use his player_sandbox.lua.