javax.servlet.ServletException: Content is not allowed
in prolog.

Check for stray non-whitespace characters outside
the document element first, as suggested by the other
answer to your question. If there aren't any, you might
have hit a Xalan bug, which was typical for the Xalan
release bundled with an older JDK 1.4 release. Upgrading
the JDK to the latest release should help.