Following the Endpoint Protection (SEP) Small Business Edition (SBE) and Cloud definitions updates on August, 2nd 2017, some systems may display a black screen or hang at the Windows welcome screen during login.
Process csrss.exe (Microsoft's Client Server Runtime Subsystem) experiences an endless wait during the logon phase due to an SDS exception caused by revision set: 20170802.020 released on August 2nd
If the system is in a functioning state; simply run the command to Update Definitions from the user interface. Alternatively, with SEP SBE, a command can be initiated to Run Liveupdate from the portal Computers tab under Perform Group Actions.
If the system is non-responsive, restart the computer in Safe-Mode and delete the SDSDefs directory to remove the affected revision.
C:\Program Files\Symantec Endpoint Protection Cloud\NortonData\*version*\Definitions\SDSDefs
Reboot the system normally and run LiveUpdate. Note: You may receive a product error, this will resolve itself once the new definitions have been downloaded.