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Clean Boot Troubleshooting Caused Windows Defender 0x80070426
I tried to do a clean boot to see if it would allow my system to sleep : Desktop computer refuses to sleep when logged in - - Windows 10 Forums
After that failed as a troubleshooting step (my system still would not sleep while idle), I tried a diagnostic startup. Upon re-enabling, I was greeted with a dialog stating that Windows Defender was unable to start with error code 0x80070426. I tried sfc /scannow, which found some corruption. I fixed that with a DISM RestoreHealth (step 7), but I still had the Windows Defender error upon startup and Defender does not start if I try to open it manually (I get the same error).
Re-enabling all services and startup programs does not fix the Defender issue, either.