In some error cases including the one above Sitebricks adds a stack trance to the response. This is a security issue with leaking information. We plug the problems we find but is there a way to tell site bricks never to send stack traces?
The problem is not at the Sitebricks level but at your servlet container or Filter level. If you are using a normal servlet container you should configure error pages in web.xml using the directive:
http://www.tutorialspoint.com/servlets/servlets-exception-handling.htm
Alternatively, you can use a filter to check if the response code is not 2XX and display a custom error page yourself.
Sitebricks shows the error trace as a debugging tool.