Have you checked your file/folder permissions? Typically a 400 response is because the system cannot figure out what kind of request is being made to it. Another idea might be to ensure you have the runaAllManagedModulesForAllRequests option set in your web.confg.
< system.webServer >
< modules runAllManagedModulesForAllRequests="true" >
This is the same setting needed for extensionless urls.