This error can happen for various reasons, the main one being a GPO that has been modified.
Try each of these following solutions:
– update RDPlus to the latest version and try again!
RDPlus is often updated and each update solves issues.
– apply all Windows patches
– open the AdminTool then go to the Home tab> click on System Audit and make sure all are green checks
– restart the RDPlus web server
– reboot the server
Then try again
Open gpedit.msc applet
– navigate to Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Security.
– then enable ‘Require use of specific security layer for remote (RDP) connections’ and select ‘RDP’ as Security Layer.
– then disable ‘Require user authentication for remote connections by using Network Level Authentication policy’.
– reboot the terminal server or use this command : gpupdate /force
Then try again.
– check RDS-Knight for blocked IPs – make sure that the RDS-Knight service is running
– check the RDPlus web server: ports must be 80/443
– check if IIS is installed and conflicting with RDPlus web server ports – delete the ports from IIS (not an issue they were already deleted)
– restart the RDPlus web server
Then try again.