Debug virtual server with SSL offloading

We have a virtual server configured. It is working fine for most clients but we have strange issues with some old systems. Intermittently we are getting "500 (Internal Server Error) unexpected EOF before status line seen". I did a packet capture and I see successful TLS handshake. The client would send a data packet right after the handshake however after about a minute there would be a "TLS alert" message back from either the FortiGate or the web server.

We don't manage the web server. We are intercepting because we need to add the "HTTP X forwarded header". 

Is there a way to debug the connection? I am hoping to see the details about the TLS handshake and also the HTTP transaction on both sides. (The server side is also SSL/TLS). It will be even better if I can see the unencrypted packet/content of the transaction.

Thanks