wait I think I got the problem
[net.minecraftforge.fml.client.ClientHooks/CLIENTHOOKS]: The client has mods that are missing on the server: [usefulbackpacks, mowziesmobs, astikorcarts, playerex, citadel, uteamcore, spartanweaponry, iceandfire, tlskincape]
but... every single of these mods actually exist on the server