Clearing your cache is unlikely the fix for the majority of 400 bad request issues, but it's quick and easy and worth trying.

A 400 Bad Request can happen for many reasons. Perhaps you've mistyped a URL and the server can't return a 404 error. Or maybe your web browser is trying to use an expired or invalid cookie.

This is being rejected with a HTTP/400 Invalid Request. i took the frontend out of http mode, into tcp mode, and the backend rejected the request before it was passed on to the pool member proxies ...

Expert Tip: Note 400 Bad Request errors, like all errors of this type, could be seen in any operating system and in any browser. How to Fix the 400 Bad Request Error Check for errors in the URL.