Can't connect to my own server with the same version and a clean install.

To Reproduce
Steps to reproduce the behavior:

1. Install server with the latest version (1.2.1)
2. Change default "startserver.bat" settings so it launches with the correct java version [Line29] in my case ""C:\Program Files\Java\jre1.8.0_301\bin\java.exe" -jar "E:\PRIVATE\Minecraft Servers\ATM 6 SKY (Flatworld)\serverstarter-2.0.1.jar""
3. Change the max ram amount to 8G in "server-setup-config.yaml"
4. Launch Server and then the client with the same version 1.2.1
5. Try to join your server using using local host
6. See the error. (Failed to synchronize registry data from server, closing connection)

Expected behavior
Should connect normally just like any other Minecraft server.

Screenshots

Additional context
I have already tried reinstalling my server fresh again and same issue, but this somehow doesn't happen to my friend. (Plus I use Optifine, but when my friend tried it was working for him with and without Optifine. He hosted his own server same setup)
Also I haven't changed the world type to flat before fully checking if the server is running fine and I can join.

Switching to MultiMC from GDLauncher seems to have fixed the issue. I was confused at first because I have never ran into this issue before with GDLauncher, but I guess the issue is now fixed. If anybody else encounters this at least they can look at this post and quickly resolve the issue.