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I have no idea why or what I have broken in the process. But here is what worked for me.

1. Kill process Explorer.exe via Task Manager.

2. Delete this registry key.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FolderTypes\{ef87b4cb-f2ce-4785-8658-4ca6c63e38c6}\TopViews\{00000000-0000-0000-0000-000000000000}

3. Start process Explorer.exe via Task Manager.