It looks like Microsoft broke Windows Defender on Windows 10 for the second time in this year. Windows Defender update, which was shipped earlier today, is causing ‘Threat service has stopped. Restart now’ error and ‘Unexpected error. Sorry, we ran into a problem. Please try again.’

While many users are blaming Windows 10 KB4549951 for Windows Defender debacle, the real culprit appears to be Security Intelligence 1.313.1638.0, which was pushed earlier today via an automatic Windows Update.

It appears that the problem is caused by files with two dots (..) in their name and the error will disappear if you rename or delete the files.

To fix ‘Threat service has stopped. Restart now’ error in Windows Defender, follow these steps

1. Open Event Viewer
   
2. In Event Viewer, navigate to Applications and Services logs > Microsoft > Windows > Windows Defender > Operational.
   
3. Look there for error and you’ll find something like “crash exception code 0xc00000005 ” and it includes the name of the file with two dots. To resolve the issue, remove the extra dot or delete the file.
   
4. Once you’ve fixed the file, go to services and start Windows Defender and perform a quick scan. This should resolve the problem.