How do I fix the "Failed to check session lock for world" error?

The session.lock file is a critical file needed in order to run a world. The error above means that this session.lock file can't be opened.

The error message says:

Stop the server and delete the session.lock in this world to prevent further issues.

But in many cases, this may not work. What worked for me was setting the permissions of the session.lock file to 777. You must have SSH access to your server (otherwise you should contact your host). Run:

chmod 777 session.lock

Make sure that you're in the world which contains the session.lock file the server has issues with.

This should make Minecraft be able to write and read the file.

If it still doesn't work, try running:

chmod 777 [world]

Replace [world] with the name of the world you're having issues with.

After moving .minecraft directory from ubuntu to kubuntu, same problem occured. For me this worked: Change owner of the 'saves'-directory from root to user's one. For instance with user sven (player's user):

christian@kubuntu-pc:~$ sudo chown sven:sven /home/sven/.minecraft/saves/ -R