We have been having an issue downloading an update from a vendor of ours over the past several months and getting a 400 error in the process. I have been working with them regularly to resolve this issue by making small changes on the firewall and our web filter. It came to be that another customer of the vendor found a solution on their Secure Computing firewall. I wanted to see how I can apply their solution on the Secure Computing firewall to my Cisco ASA 5520. Here's an insert from the email:
Any help would be appreciated. If this solution does not apply, then no worries...it's back to the drawing board.
Anyway, this morning I configured the firewall to use an IP Filter rule, which instead of proxy, will securely and directly forward IP packets between the client and the ip address of company(x.x.x.x). I suspect that some of your other clients that have a similar issue may use this type, or even brand of firewall.