If you get this error message: 

Then your connection client is corrupted.

Run C:\Users\USERNAME\RDP6\unins000.exe before deleting RDP6 folder located in C:\Users\Username

On your server you need to re-install this file : http://dl-files.com/Setup-RemoteAppClient.exe

And on the end-user's workstation, you will also need to install the same file : http://dl-files.com/Setup-RemoteAppClient.exe

AND on each user profile too. That is mandatory. 

Then, when installing the Web plugin on the end user's workstation, make sure you check the "for all users" option!

Please also check this other FAQ :