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