After making a change and running a sanity check, Nagios returned with "No space on this device". I'm not sure what this means or what I can do about it.
See what command df -h shows and clean disk
Figured it out after a while. HDD was full, all 100%. I had splunk installed and the damned this never cleans out its database files. I remove all the splunk files and uninstalled it, subsequently freeing up 70% of the HDD.