Failed to check session lock minecraft server

What this actually means

Another process already holds this world. Either the server is genuinely still running, or a previous run did not shut down cleanly and left the session.lock held.

How to fix it

Make sure no other copy of the server is running -- check Task Manager for a java process, or `ps aux | grep java`. Stop it properly with the `stop` command, never by killing the window, or the lock survives. NOTE: if you are boot-testing a server while someone is playing on it, THIS is the error you get, and it tests nothing. Stop the live server first or test against a copy of the world.

How to be sure it is this

Look for a running java process holding the world. On Linux ps aux | grep java; on Windows check Task Manager for javaw.

What it gets mistaken for

A crashed server that did not clean up looks the same as a second server genuinely running. The process list is the only way to tell.

What to do next

Kill the process, then start. Delete session.lock only once you are certain nothing holds it — two servers writing one world corrupts it for real, and that is not recoverable.

What it looks like in your log

Search your crash report or latest.log for any of these:

Failed to check session lock
The directory  ...  is already being used
MinecraftException: Failed to check session lock
world is already in use

The free edition of Forge Server Doctor detects this one. It is a single Python file with no dependencies — point it at the report and it names the cause and the mod jar:

python3 forge_doctor.py crash-reports/crash-....txt