Bug/Issue Friend can't connect to dedicated mod server?

Code:

(EofException) Failed to read full buffer in readFull, eof reached.

My client log(if it's important)
Server logs

He sees his quickbar and the space background in hyperspace for a split second before disconnect if he skips the intro and nothing at all if he doesn't.

If you need the actual text document tell me and I'll upload it.

I've tried these things in many different orders and combinations:

• Deleting planet they are orbiting
• Deleting their ship
• Deleting the universe folder
• Him starting singleplayer to play the game (works fine) and then joining the server after (crashes to title with EOF)
• Using steam to connect via invite in singleplayer (Just crashes both of our games completely instead of to title)
• Creating a new character
• Restarting PC
• Remodeling the mod pack to not include mods that tend to cause an EOF in multiplayer

Mods used

If at all possible I'd like to become more familiar with the process of troubleshooting this sort of error. All leads are welcome!