Microsoft’s Windows Defender signature file that was causing system file checks to fail got a patch this week. But the patch caused an even bigger issue, making Defender user-triggered antivirus scans fail altogether. The System File Checker (SFC) had been out of commission since July. Real-time scanning was still enabled; and the Custom scan option, where users can choose the folders they want to be checked, was also still working. The issue has been resolved in the Security Intelligence Update for Windows Defender Antivirus KB2267602.
Source: https://threatpost.com/windows-defender-failing-microsoft-silent-update/148513/