When performing a full antivirus scan using Windows Defender, a recent definition update or Windows update is causing the program to crash in the middle of a scan. The problem is caused by the Windows Defender Antivirus Service service crashing, which will leads to a cascading series of errors displayed in event viewer and Windows Security. Users will see the crash listed as an “Application Error”” or an APPCRASH as shown below. The only way to stop these errors is to go into the Service Manager and start the. Windows Defender. Users can manually update Windows Defender definitions by clicking on the ‘Check for updates’ link.”
Source: https://www.bleepingcomputer.com/news/microsoft/windows-defender-broken-by-recent-updates-how-to-fix/