I recently rolled out a Citrix Xenapp 6 farm with a published application which was a desktop. When users started to logon they began getting the error in the above image.

I was using HPT5550 Thin Clients. So I flashed the thin clients with the latest firmware which was released in April 2011 and also updated the ICA Client to 11.02.I also noticed that some standard hot keys like alt-tab , ctrl c , ctrl v were not working.

So the simple fix is to untick “view in seamless windows” on the thin client ica connection.