This error occurs in two scenarios:
1. If the server is already running.
2. If the server is not properly killed and at times it does retain some references to the .lok file under the tmp directory of the DOMAIN/server folder and the lok file is not relinquished totally.
When the server starts it checks if any .lok file in present in the tmp file.
If yes, it would try to acquire a lock on that one.
If the lock is already held by some other process, then it would throw this error.
Check the running processes which can acquire lock on the .lok files and kill them.
Restart the server.