KB5012643, an optional preview update for Windows 11 released in April 2022, includes an annoying bug that prevents entering Safe Mode without networking support. It makes the screen flicker because of explorer.exe crashes in a loop. Microsoft resolved this issue on April 29.
Due to crashes of the Explorer process, most components that rely on it, including the Start menu and taskbar, are unusable. The crashes of the shell are logged as a "System error" on the Windows Event Log, with Source “Winlogon” and the following description: “The shell stopped unexpectedly and explorer.exe was restarted.”
As a workaround, Microsoft suggests starting Windows 11 in Safe Mode with Networking. The issue doesn't affect this mode. But the regular Safe Mode without Networking will have explorer crashes.
Check out how to Boot Windows 11 in Safe Mode.
On April 29, Microsoft has fixed the issue by using Known Issue Rollback. This a special mechanism that allows Microsoft to remotely disable the problematic code changes in Windows and restore the previous stable software. The switch between the old and new code happens transparently for end user. However, in some cases users need to restart their devices.
For the Safe Mode fix, Microsoft noted that a restart may speed up the resolution delivery to the consumer device. Otherwise it might take up to 24 hours to apply So if your PC is running up 24x7 or you are using Fast Startup or Hibernation, it is better to restart Windows 11.
Safe Mode is a special boot mode Windows provides for troubleshooting. It is mainly used when the OS cannot start normally. Therefore, if your computer is running without problems, you are unlikely to notice the bug.
You can more details on the Windows 11 known issues website.
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