As Mr. Boots said, it may be a disk error. Alternately, some files Defender requires may be corrupted.
- Disable Windows Defender in Settings tab under Administrator section
- Delete the folder C:\ProgramData\Microsoft\Windows Defender\Scans
It may be hidden, and will require elevation to access it.
- Enable Windows Defender using the Security Center notification on your task bar
When you enable Defender, the files should be recreated.