UPDATE 2: Problem recreates itself, so I dunno anymore... Looks like its just a temporary fix...

UPDATE: Turns out me and my friend's player data was mixed up, so the world thought we were the same player (same inventory). When we would get close to each other the world would crash. What I did is download a Universal Minecraft Editor and deleted all the player data. We lost our inventories, but the world is ok and no longer crashing!
The editor: https://www.universalminecrafteditor.com/

This only started to affect one of our worlds (nothing wrong with other worlds). It was created in 1.2.20.2 and now if a second player tries to join, the host crashes as soon as the second player is on the "Locating world" screen. World seems to work fine in singleplayer, so hopefully there is a way to fix it to re-allow multiplayer!

LINK TO WORLD DOWNLOAD: https://drive.google.com/open?id=10VPMc7gGBtgJmtAz4VAA2qfMJ3VoL2ab