I had a problem accessing any RDP application on my UAG portal (whether its Remote App application , RDP User defined, RDP Gateway or VDI Pool– Predefined Remote Desktop Host), after several hours and investigation with Microsoft team, it turned to be an update released more than 2 weeks ago KB2592687 regarding RDP 8.0 update.
Looks like UAG Portal is not yet compatible with the new RDP 8.0 update. After uninstalling/Removing this update from my client machine (Windows 7), everything worked fine. I would recommend everyone using the UAG portal to hold for this update till its fully compatible. I escalated this issue for further investigation.
For Corporates with WSUS implementation, they may decline/Disapprove this update till further update.