The world could not be opened
What it looks like in your log
Failed to check session lock for world located at ./world, aborting. Stop the server and delete session.lock
What causes it
The world is open in another server, or the save may be damaged.
How to fix it
Close any other copy of this server, or restore a backup from the Backups section.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log