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Windows computers sometimes hang on startup after installing Symantec Endpoint Protection 12.1 RU4 MP1 or newer
This issue occurs when Symantec Endpoint Protection's Auto-Protect component does not properly release its opportunistic lock (oplock).
Disabling Deferred Scanning for Symantec Endpoint Protection's Auto-Protect component on affected machines
Follow the instructions in knowledgebase document TECH224108 - https://www.symantec.com/docs/TECH224108