I have contacted Fortinet support and have done some research of my own... This may have fixed the issue, but I need to do more testing to be 100% sure... Its an intermittent problem for me (4.1MR3).
I produced many debug reports for Fortinet, but nothing showed, they believe one of the two solutions below is the answer.
ticket below:
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Dear Support,
We are planning to release SSL RDP to approx 100 users over the next few weeks. However, I have noticed that occasionally the JAVA RDP client drops connection to our Windows 2003 Terminal servers and displays following error message " Connection terminated - Internal Protocol Error" (Figure 1), and the server event log reports " The RDP protocol component DATA ENCRYPTION detected an error in the protocol stream and has disconnected the client" (Figure 2).
It' s worth noting that we don' t experience these connection issues when using the native Windows RDP client (using IPSEC or SSL Tunnel Mode), so it appears that there may be an issue with the Fortinet JAVA RDP client or how the data is encapsulated.
There is a Microsoft knowledge base article (KB323497) that explains the symptoms and a possible registry fix (not tested yet).
Another forum suggests disabling " large send offload" on the NIC as this can interrupt the RDP stream (I am currently testing this solution).
The Fortinet forum http://support.fortinet.com/forum/tm.asp?m=57360&p=2&tmode=1&smode=1 also suggests that other users are having similar problems.
Please can you help?
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It would be useful if someone else can try the above and compare results.
Regards - Rob