I have BIRT viewer (v2.5.1) deployed on Tomcat 6 server behind firewall. Application server retrieve reports one by one from BIRT viewer and includes them in to output page. That part is working perfectly

Now, I tried to show images included in report to end user. Because user can not access them directly, I am using Apache on application server to proxy requests to server on which BIRT viewer is running. Those request comes to BIRT viewer exactly same like in case I am running viewer directly, but instead of getting image, I got error 500 with following stacktrace:
java.lang.NullPointerException
at org.eclipse.birt.report.servlet.BaseReportEngineServlet.doGet(BaseReportEngineServlet.java:172)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at org.apache.axis.transport.http.AxisServletBase.service(AxisServletBase.java:327)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.eclipse.birt.report.servlet.BaseReportEngineServlet.service(BaseReportEngineServlet.java:116)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.eclipse.birt.report.filter.ViewerFilter.doFilter(ViewerFilter.java:68)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:568)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:857)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:565)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1509)
at java.lang.Thread.run(Thread.java:619)
Any ideas?