Your license allows more than 2 users, however TSplus allows no more than 2 concurrent connections.

1 - Please update your TSplus to the latest version, reboot the server and try again.

You can get the update here :

Make sure no users are logged in before installing this update, you can check for remote users by launching a task manager and clicking on the users tab. Disabling your anti virus is also recommended. 

Make a scan exclusion rule on C:\wsession and C:\Program Files (x86)\TSplus\UserDesktop\files and make sure your users have sufficient rights to read and execute all programs in these folders.

2 - If it still doesn't work, then it means that a Windows update has impacted the RDP connection module and has overwritten the TSplus settings.

- open a DOS command prompt as administrator

- navigate to the tsplus\userdesktop\files installation directory (cd "c:\Program Files (x86)\tsplus\UserDesktop\files")

- run 'svcac.exe /updatekernel'

- after the popup, open the Windows Services Manager
- open the Wndows Service Manager

- restart the 'Application Publishing Service (APS)' service. Two services will also be restarted

After that you should return to normal operation in multi-session mode.