Hi Gary,
The particular error code you are seeing is essentially telling you that a scheduled maintenance scan (which would include Windows Defender by default as you have chosen to use this as your primary security application) was interrupted and thus the scan either did not begin, or halted part way through a scan.
The Maintenance Scans require an idle system state. If you happened to be using your computer at the time such a scan was scheduled or had started then the system state is now no longer idle, and thus the error is triggered.
Perhaps you have chosen to set up a scan in your startup options, or you actually created a new Scheduled Task. If you did then I'd recommend removing this as you are actually duplicating a scan that will automatically take place anyway. Duplication in this instance could cause Defender to attempt to scan twice which would create a non idle system state and cancel both scans - thus defeating any security gains.
It is sufficient to allow Windows to maintain itself through its standard Maintenance, and for you on occasion to run a manual scan yourself when you have time to sit back and let it do a complete scan. If you cancel a scan for some reason yourself, this too will trigger the same error code.
Another possibility is that you have a second antivirus security program installed, for example, AVG, Norton, Avast, McAfee, etcetera.
Any one of these will schedule their own System Idle scans, and if they happen to occur at the same time Windows Defender is doing its own System idle scan, you guessed it, Error 0x8007042B is triggered.
If this were the case for you I'd recommend choosing one or the other and disabling the one you choose not to use.
If you want to read more about this:
Regards,
Regedit32