Using Windows Server 2008 R2 Remote Desktop Services RemoteApp.
Error: “ActiveX control installed or not”
“The Remote Desktop Services ActiveX Client control is not available…” (Screenshot below)
While I found a lot of blogs describing registry keys to clear and the like, ultimately the simplest solution proved to be the most effective with this error. Simply resetting Internet Explorer to the default settings has proven to consistently clear this error.
Microsoft article for resetting IE: http://support.microsoft.com/kb/923737