Before Continuing:

• Check you are using the latest version of the mods and its dependencies
• Always include a latest.log if you are crashing
• Remove mod that enhances Minecraft: Optifine, Sodium, others. The issue persists.
• If you are unsure which mod is the culprit.
  Disable all of your mods and enable them 1-2 mods each time to isolate the culprit
• Confirm that there is no existing issue with a similar description submitted in the list of issues.

Version

1.21.1-NEOFORGE

Supplementaries Version

neoforge_1.21-3.0.10-alpha

Moonlight Lib Version

neoforge_1.21-2.16.0

Issue Detail

disconnect-2024-11-11_19.04.04-client.txt
latest.log

Game loads fine, when attempting to load into or create a world it errors out with "Connection Lost, Network Protocol Error"

The same happens with Fabric on MC 1.21.1, singleplayer.
It happened when I took a map of a pillager outpost from a chest and put it in my inventory. After which this world can only be loaded if the supplementaries mod is removed from the "mods" folder, because "connection lost" right after the world is loaded.
I think this problem is caused by the mod's code that deals with multiplayer/network.
Maybe everything network-related should be disabled when in singleplayer, automatically or through a config option.

duplicate of #1195