Maybe the problem resides on the response.sendRedirect("") you are using.
Is there any reason to use the response.sendRedirect("") instead of the RequestDispatcher.forward(request, response)? I mean, it seems you are using it inside the same web application, so I find more suitable to use a forward than a redirect.
Search information about these two methods and maybe here is the problem. Check out this link...
You guys are awesome! Due to my ignorance I thought you always had to supply an absolute url to response.sendRedirect. I change code as Charles suggested and it worked. I'm sure Raimon's way would of worked also.