DISCLAIMER: If you are not using the sslvpn web portal the following may not make sense.
This morning our sslvpn users called saying that they could not launch the RDP sessions from the web portal. After much investigation my colleague and I narrowed it down to a recent Java update. Apparently after this Java 7 update 51 the security settings are more restrictive. If you use the sslvpn portal you may want to check the security settings on your clients. I have attached a screenshot.