Windows Defender not running scans

I wonder if anyone can identify why Windows Defender has suddenly, i.e. the last day or two, stopped running scans on my home PC?

The product is up to date:



If I attempt to scan a folder of file, the screen above appears and sometimes the scan will run, but more often no scan is performed and the screen below is displayed:



There seems to be no pattern to this erratic behaviour.

I use MalwareBytes and CCleaner in real-time mode so I wonder if they might be affecting Windows Defender at all. However, if both are switched off the same problem occurs.

I have Windows 10 build OS 17763.18.

Thanks for your response Ken.

I updated my version of Windows 10 a few hours after my original post. It is now version 17763.168.

Windows Defender appears to be behaving more normally but I will be keeping an eye on it.

I do wonder if some of the more recent Windows updates may have had some side effects of the nature I described?