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Q: When I try to join this Win2012 computer to the domain, it gives an error I’ve never seen before:

“The following error occurred attempting to join the domain “”:
The request is not supported.”

A: This server was once a domain controller. During demotion, something prevented the removal of the following registry value name:

DSA Database file

Delete that “Dsa Database File” value name and attempt to join the domain again. It should work this time. If you take a gander at the %systemroot%\debug\netsetup.log, you’ll see another clue that this is your issue:

NetpIsTargetImageADC: Determined this is a DC image as RegQueryValueExW loaded Services\NTDS\Parameters\DSA Database file: 0x0
NetpInitiateOfflineJoin: The image at C:\Windows\system32\config\SYSTEM is a DC: 0x32