Short but sweet this one - a way of troubleshooting connectivity problems between analytics (the Presentation Services Plug-in, either j2ee servlet or ISAPI, a.k.a. SAWBridge) and sawserver (Presentation Services).
For a recap on the services & flow please see the first few paragraphs of this post.
Problems in connectivity between analytics and sawserver normally manifest themselves through this error message:
500 Internal Server Error Servlet error: An exception occurred. The current application deployment descriptors do not allow for including it in this response.