Server Manager automatically grants the Authorization Server access to the SPFUsers group when you configure a new site and gives the SPFUsers group Read & Execute and Modify permissions.
You can manually grant access for an existing site by:
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Navigating to: [Windows Drive]:\ProgramData\Microsoft\Crypto\RSA.
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Right-clicking the MachineKeys folder.
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Give the SPFUsers group Read & Execute and Modify permissions.
If the [Drive]:\ProgramData folder is hidden, enable the Show hidden files, folders, and drives system folder option.