Recently we started receiving calls from our Fortigate customers that they cannot launch RDP sessions from within their web portal. After a bit of troubleshooting I discovered that this was caused by a recent Java update (Java 7 Update 51). More information can be found here, here and here.
The exact error looks like this:
The workaround in this case is very simple, just add your Fortigate Portal address to the Exception Site List in your Java configuration, like so:
More information about the exception list can be found here.