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“Failed to check session lock” is an error that randomly occurs when the Minecraft server can’t access the session file, causing the server to crash or prevent it from starting up. Luckily, the fix is really simple!
The error: net.Minecraft.server.WorldConlictException: Failed to check session lock, aborting.
To fix the session lock error, head to the Fractal dashboard and do the following steps:
  1. Stop the server
  2. In the file manager, open the world folder
  3. Delete the session.lock file
  4. Start the server
This will regenerate the session file for you.