Ars Instrumentum is improperly configured and basically doesnt verify it is actually on both the client and server, allowing a client that has the mod to join a server which doesn't.

The end result is that clients crash when they trigger a key press event, somewhat randomly but very quickly after joining.

I'm assuming thats just a tag / property that isnt properly setup in some forge config to tell the mod is required on both sides.

That's intentional, so people who must have mana numbers can, even when the server doesn't have the mod.
Just rebind the Armarium keys to unbound.