Okay, your post before last was helpful. You indicate that, after logon, any link you click generates the same exact POST request and returns back to the home page. At this point it's very likely that the application logic doesn't like what it's getting from the proxy and is trying to get the user back on track. Your best bet then into compare the initial requests and responses to the site, up to and including this POST request in both the proxied and non-proxied environments.
You didn't answer whether the virtual server was using an iRule or any special configuration(s).
Also, if you turn off client side SSL on the BIG-IP does it make a difference?