Session Lock Failed
The server fails to load a world with a session-lock error. Despite the wording about players, this almost always means the world is already opened by another process, or a previous run left the lock in a bad state.
What does this error mean?
Minecraft writes a session.lock file in each world to ensure only one instance uses it at a time. 'Failed to check session lock' means that guard tripped, typically because two servers point at the same world, or a crash left the lock behind.
Failed to check session lock, possibly too many players
Most Common Causes
- Two server instances using the same world folder.
- A previous instance still running in the background.
- A crash that left session.lock in a stale state.
- A shared/synced folder accessed by multiple hosts.
- Filesystem issues preventing the lock from updating.
How To Diagnose
- Check for another running server using that world ('ps aux | grep java').
- Confirm no second instance or panel server points at the same folder.
- Look for a lingering process after a crash.
- Verify the world is not on a folder shared by multiple machines.
Recommended Fixes
- Run only one instance
Ensure a single server uses the world, stop any duplicate or leftover process. - Kill the old process
After a crash, end the lingering Java process, then start fresh so the lock is recreated. - Avoid shared world folders
Do not point two hosts at the same world directory. - Restart cleanly
A clean restart regenerates session.lock correctly.
Frequently Asked Questions
No, despite the wording it almost always means the world is open elsewhere or the lock is stale.
Only after confirming no other process uses the world, the server recreates it on start.
An unclean exit can leave the lock behind, restarting with no duplicate process resolves it.