I'm using the same setup. Can you please provide some more information - eg your directory structure for you webapp etc. Just use fake names if you don't want to give it all away.
I'm having a similar problem.
If I execute s servlet with a servlet name Test - http://localhost:5678/Test and I have response.sendRedirect("?page=value"); the server returns error 500 and the URL looks like this: http://localhost:5678/?page=value.
If I manually add Test - http://localhost:5678/Test?page=value the page loads fine. One of the solutions was to add request.getRequestsURI() in sendRedirect. It worked but need to do that without changing the code, probably it is a configurations issue with Tomcat that I don't know how to resolve.
The application works ok with iPlanet using Servlet Virtual Path Translation - mapping @.*html to Test. That is somting that I don't know how to do in Tomcat. I tried
but it didn't work for me.
Any help will be appreciated.