Any changes to the environment or Security Settings specific to RDP? Try the below settings:
Disable NLA (regedit) and enable PKU2U (gpedit.msc) AND REBOOT:
Set "UserAuthentication" to DWord value 0
Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options
Set "Network security: Allow PKU2U authentication requests to this computer to use online identities" to Enabled
Hope this helps.
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