Just tried installing the standalone server & when I attempt to join I get the error message -

"Incomplete set of tags received from server.
Please contact server operator"

I've searched online and can see that it's generally something to do with the modlists not being the same between the servers, but I deleted all the mods in the server, copy pasted all the ones from my personal directory over to no avail.

Is there a good solution to this?

Was running minecraft-server.jar not the forge one. I'm smart.

Leaving this here to hopefully help someone else.