Citrix Xenapp 6.5 Server with CSG - HTTP 400 Bad Request error

The BIG-IP can easily be used to replace CSG function, however, here is a pretty good whitepaper on CSG setup

400 (bad request) errors can be caused by XML server requests failing, I would double check to make sure you are using the correct ports for your xenapp (xml broker) connection.

Might be that you need to add a host entry to the URL being used to access your external site (page 40 of white paper), where your host entry would have the url pointing to the localhost.

The other advise I can offer is to run a tcpdump on your server to watch connection attempts to see where the failure occurs.

If all else fails, you could open a ticket up with Citrix support.