Symptom:

Terminal-Services-ClientActiveXCore Event ID 226 on RDP Server:

rdpclient_ssl: an error was encountered when transitioning from TsSslStateDisconnected in response to TsSslEventInvalidState (error code 0x8000FFFF)

Error message from RDP client after 10+ seconds of displaying “Securing remote connection…”

Resolution:

Uninstall RDP Client

Get-AppxPackage -Name Microsoft.RemoteDesktop | Remove-AppxPackage -Verbose

Reinstall RDP Client

Get-AppxPackage -Name Microsoft.RemoteDesktop | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml" -Verbose}