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Recently I had an issue where I was getting the following error when trying to launch a Virtual Desktop via a CAG.
Upon looking closer at the issue I noticed that the Web Interface config had not been setup right and the URL for the STA was actually missing the correct path (http://IPAddressOfDDC/scripts/ctxsta.dll) to the STA and once it was changes to the correct format the Virtual Desktops could launch successfully.
If you have the same issue check the event logs of the Web Interface and see if event ID's 30016 and 30205 are being logged.
Hope this helps someone out.