This error can happen for various reasons, the main one being a GPO that has been modified.

Try each of these following solutions:

- update TSplus to the latest version and try again!

TSplus 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 TSPlus 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 TSPlus web server: ports must be 80/443

- check if IIS is installed and conflicting with TSPlus web server ports – delete the ports from IIS (not an issue they were already deleted)

- restart the TSPlus web server

Then try again.