This issue occurs due to the insufficient space available on IPS Sensor.
For a workaround,
Clear out the temp update folder in order to create some space for the signature upgrade.
Use these commands from the service account:
#rm -rf /usr/cids/idsRoot/var/updates/files/S69#rm -rf /usr/cids/idsRoot/var/updates/files/common#rm /usr/cids/idsRoot/var/virtualSensor/*#rm /usr/cids/idsRoot/var/.tmp/*
Refer to the Software Download IPS 5.x page for the latest service pack downloads.