This issue is due to the presence of Cisco bug ID CSCsc92316 and occurs most often in the Intrusion Prevention System (IPS) version 5.1(1).
The IPS almost always encounters this issue when these conditions are met:
The temporary workaround for this issue is to reboot the sensor.
In order to completely resolve this issue, upgrade the IPS to version 5.1(2) or later. Refer to Cisco Downloads in order to download the latest software version.
Refer to the Upgrading the Sensor section of Upgrading, Downgrading, and Installing System Images in IPS.