Works Great in a single player game with all the same mods and everything. But when I try connecting to my dedicated server, it does the handshake and shows I disconnected.

[22:45:04] [User Authenticator #1/INFO]: UUID of player Korg3471 is ********-****-****-****-************
[22:45:05] [Server thread/INFO]: Korg3471[/10.1.0.45:36564] logged in with entity id 287 at (8.5, 73.0, -4.5)
[22:45:05] [Server thread/INFO]: [Time & Wind] Sending config to player
[22:45:05] [Server thread/INFO]: Sending server config values to client
[22:45:05] [Server thread/INFO]: Korg3471 joined the game
[22:45:06] [Netty Server IO #2/INFO]: Receiving client config
[22:45:06] [Server thread/INFO]: Korg3471 lost connection: Disconnected
[22:45:06] [Server thread/INFO]: Korg3471 left the game

The client spit out:
[22:45:13] [Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IllegalArgumentException: duplicate key: net.minecraft.class_1856@70dcb7e4

I left in Alloy Forgery and it all loaded fine, so there's something in MM that is preventing connections or duplicating something. Even a single player game shared to the LAN has the same error for anyone trying to connect while the host can chug along like nothing is wrong.

There is some bug with Alloy Forgery which is causing recipe syncing to fail. I am looking into the issue

We have identifier the issue and are working on a fix. Some context surrounding the issue:

The cause of the issue is due to a faulty implementation of recipe syncing in Alloy Forgery. The ingredient handling doesn't implement a check (hashcode), which causes it to fail in multiplayer.

In theory I would be able to make a fix on Mythic Metals end by changing up all the recipes. It doesn't fix the core problem though, as any other mods/modpacks would still break.

Can confirm latest fix allows server/lan play again! Nice work!