Several minutes after logging on to a Windows 10 RS3 (version 1709) system running Symantec Endpoint Protection (SEP) 14 RU1 (MP1), a "Windows Defender Security Center" window appears, stating "The threat service has stopped. Restart it now." Additionally, a message stating that "Windows Defender Antivirus and Symantec Endpoint Protection are deactivated" is shown. When you click the "Restart now" button, nothing happens. In the "Windows Defender Security Center" window, the "Open Symantec Endpoint Protection" button cannot be clicked.
The issue resolves itself after several minutes.
SEP 14 RU1 (MP1)
This issue was resolved in Windows 10 RS4 (version 1803), which is supported by SEP 14 RU1 MP2.