Without seeing or knowing how the receivers were set up, we can't troubleshoot it..
1. Look to make sure you are dual path to each receiver..
Turn off alarm and unplug each one and see..
2. Check failsafe for each receiver to see if its disabled.. iF not, then you are done... The CBox switched to one of them and it lied to it all the way to the ground.
3. Look at the log data to see what the signal strength was for each receiver at the time of the incident.
See if you can determine which receiver or both gave up the ghost..
4. post your findings and alarm settings