Hello, and welcome to the community.
Please go to the Devices page of your Network Security Manager, and select the problem device. Dig down to Troubleshooting > Attack Compilation. Please disable all custom attacks, and press 'Save'.
This will tell the Manager, when pushing updates to that Sensor, to not push any custom signatures that you may have on the Manager. Sometimes custom signatures interfere with the Sensor update.
Please click the 'Deploy Pending Changes' and attempt to push an update to your Sensor.
If this works, custom signatures are the issue. If this does not work, report the results and we'll do further troubleshooting.
Technical Support Engineer
McAfee. Part of Intel Security.
this is known bug in some 7.1.x sw. version, because new signature set requires more memory. Upgrade to latest software.