There are several Win7 Ultimate endpoints configured with WMI sensors. They will work initially but over time the same sensors will fail with "Connection could not be established (code: PE015)". Researching this error directed me to run the Paessler WMI Test utility against the affected endpoint. I performed this on the host running the probe. The results seem to show normal results.
Restarting the server or PRTG will clear this condition however as before, over time the problem will occur again.