Updated to build 14371 yesterday - no problems.
Booted to Insider Preview today and get error message: Windows Defender PC Status: At risk - Your PC isn't being monitored because the app's service stopped. You should restart it now. Clicking the big red Start now button - result "The service couldn't be started" Close
Administrative Tools > Services:
Windows Defender Advanced Threat Protection Service is Startup Type = Manual; selecting 'Start the service' resulted in Error 13: The data is invalid - OK
Windows Defender Network Inspection Service is Startup Type = Manual; selecting 'Start the service' resulted in Status = Running
Windows Defender Service is Startup Type = Automatic, Status is blank; selecting 'Start the service' resulted in Error 0x80070005: Access is denied - OK