The SEP client service stops after a GUP proxy WinHTTP URL request related string operation
Article ID: 163932
Symantec Endpoint Protection 12.1
The issue was resolved in Symantec Endpoint Protection 14, which did away with the use of WinHTTP as a transport framework.
Changing GUP throttling to unlimited may or may not resolve the issue in Symantec Endpoint Protection 12.1.