I have a couple of QoS Round Trip sensors. Occasionally the sensor will fail with the error: "Could not open UDP Port xxx. Please make sure this port is not in use. (code: PE091)" Restarting both probes does not help. The only workaround I found is to change the port each time it happens and it fixed the sensor for a while until it happens again.
The ports I'm using are completely random and not used by any other service, also it works for several days fine before it fails.
PRTG version 126.96.36.1992