Version 4.18.1908.7 of Windows Defender has a critical bug that causes the Quick, Full antivirus scans and also scans you scheduled are not performed.
Microsoft released a fix for the issue with a Windows defender definition update KB2267602 (Version 1.301.1684.0).
To check if computers or server in your network are affected, you can use the following report: https://www.lansweeper.com/report/windows-defender-antivirus-broken-scan-audit/
