Anyone with a hosting service from Site5 could have this problem: when you have your web application in a directory symlinked, like
~/public_html/webapp -> ~/apps/webapp
when accessing to the URL http://yourdomain.com/webapp you will have an error (Bad Request). But if you add the trailing slash to the URL, everything will work fine. After some digging in Site5 foruns, I resolved the problem by creating a file, named 400.shtml, placed in the public_html directory, with the following content:
Hope it helps.