Certain player can't join certain world

Hi, I made a dedicated server for Minecraft Bedrock and everything went well with my friends (who all play on windows) but since a couple of days one of my friends can't join anymore (kinda).  Let me explain: When my friend joins the world his player is standing in the world and can pick up items but the message "username joined the game" never shows in chat and my friend's screen is just saying "locating server". When he clicks cancel on his screen the message "username left the game" DOES pup-up. so he joins the server but also doesn't...

First we tried the basic things: restarting his device, reconnecting to internet etc.
But all those things didn't work so I tried a few things in the server itself and came to the conclusion that when I change the "level-name" (changing the world) to a different world the player joins with no issues.

I've attached the level.dat and the issue occurs at this player:

I have no idea how to recreate this but if anyone has a solution that would be great!