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.
Best Answer
First, next time please include more information about your question, such as the server you are trying to join, or general PC specs - OS etc, or if your game is modded.
To troubleshoot the problem, temporarily disable the firewall. Control Panel > System and Security > Windows Firewall (or just type in “Firewall” in the Start Menu search box)
Then, on the left disable the firewall "Turn windows defender firewall on or off" - Try to join the server with this off.
IMPORTANT: Turn firewall back on after testing - Leaving off can allow malicious programs to gain access to your computer.
If this solved your problem, you can whitelist (allow) minecraft to bypass the firewall - "Allow and app or feature through Windows Defender Firewall"
You will need to allow Java(TM) Platform SE Binary
If that doesn't work, also try and whitelist the minecraft jar that you play on.
Do this by finding "Allow another app..." then navigating to your minecraft folder. (Browse > Windows Drive > Users > Your user > Appdata > Roaming > .minecraft > versions, then click into the folder of the version you play, and whitelist 1.XX.X.jar (eg: 1.13.2.jar)
If this doesn't work, there are many other reasons that can cause this. It could be server-side - You should contact server administrators, or your ISP could be blocking your traffic.
Feel free to reply/edit