When I attempt to join a server with this mod installed, I get this error
Optifine is installed on client but this might be a mod incompatibility since this didn't happen until recently, I just cannot find the mod that's causing this

Double check server and client have the same version

They had the same version

Is this fabric or forge?

forge 40.2.1

And when you remove aircraft from the client, can you join? Also please send me the exact file name of the jar on server and client.

nevermind, somehow the server ended up with 3.2.1 and the client had 4.2.1. Not sure how this happened since im using the curseforge launcher and they had the same versions at some point