These errors generally happen when the sensor has generated a new certificate (like after a re-image, or a version 4.x to 5.0 upgrade).
There is a client still trying to connect to the sensor, but has the sensor's old certificate saved away.
This generaly happens with IEV or Security Monitor (within VMS).
How to track it down:
Create a service account.
Login with the service account.
Switch to user root (su -) using the same password as the service account.
Run "ifconfig -a" to determine the interface with the sensor's IP assigned to it.
Execute "tcpdump -i "
Look for what IP Addresses are attempting to connect to port 443 (HTTPS) of the sensor.
Track down these IP Addresses and ensure the software running on these IPs has been updated with the sensor's new certificate.