IllegalStateException: Accessing LegacyRandomSource from Multiple Threads

What the log looks like

java\.lang\.IllegalStateException: Accessing LegacyRandomSource from multiple threads
Description: Accessing LegacyRandomSource from multiple threads

How to fix it

1. Open the full crash report file and scroll down to the `-- Thread dumps --` section; ignore the `Server thread` or `Render thread` stack trace and inspect the other thread dump to see which mod or plugin was running in the background.
2. Look for non-vanilla package names (lines that do not start with `net.minecraft.` or `java.`) in that background thread dump, then update or remove the matching mod or plugin.
3. If you crash on the client while Roughly Enough Items (REI) is still initializing (especially alongside KubeJS), wait for REI to reach 100% before opening or closing your inventory, or update REI and its compatibility plugins.
4. If you use experimental multithreading or async tick optimization mods, disable them temporarily to confirm whether off-thread world access is causing the crash.
5. On Paper, Spigot, or Purpur servers, check recently added plugins that use `AsyncChatEvent` or async schedulers to play sounds, spawn entities, or touch worlds off the main thread, and report the trace to the plugin author.

Check your own log