A 400 Bad Request error occurs when a request sent to the website server is incorrect or corrupted, and the server receiving the request cannot understand it.

A 400 error that's reported for a link within a Microsoft Office application will often appear as a The remote server returned an error: (400) Bad Request. message within a small pop-up window.

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