YOu can stop splunk and run ' splunk clean locks' and that should fix this. However this is typical related to starting Splunk without proper permissions, or as a different user, and then restarting as the original user.
Another potential issue could be open file limits on the linux side.. not sure on the windows.
I was getting similar error while trying to start the splunk service - "Operation "ospath_fopen" failed in /opt/splunk/src/libzero/conf-mutator-locking.c:338, conf_mutator_lock(); Permission denied" and the service start was failing.
Following your suggestion, tried - "<splunk_home>/bin/splunk clean locks" and tried starting the service. It worked without issues; service was started, and the Web UI came up as well. Many thanks to you.
Splunk Enterprise Version: 9.0.4.1
Had same issue. Running powershell/cmd as admin user worked for me.
Also same issue with linux, version 5.0.3 universal forwarder
Operation "ospath_fopen" failed in /opt/splunk/p4/splunk/branches/5.0.3/src/libzero/conf-mutator-locking.c:254, conf_mutator_lock(); Permission denied