Q: Tomcat IllegalStateException: org.apache.catalina.connector.Response.getWriter

If you get an error like this (need not start with apache, that depends on your servlet container), it is actually not your real error. To get the real error, turn on Verbose Exceptions by going to Config -> Server and hitting the check box (be sure to submit and apply). This is due to the nature of streaming results back, if an error occurs after GeoServer has started returning results then this is what the client program sees. This error should hopefully fade away, as the 'partial-buffer' strategy of returning results should catch a lot more cases.

The most common source of this error is due to not having the xalan jars, see the faq question on Java 1.5