We have installed the Jenkins on Tomcat (version 6.0.18).
After upgrading Jenkins from version 1.522 to 1.537 every time a user connects to the server or clicks any link on the Jenkins GUI a null pointer exception appears in the log (see below).

After some investigation I found that this issue was introduced with the Jenkins version 1.526.

04.11.2013 07:00:06 org.apache.coyote.ajp.AjpMessage appendString
SEVERE: Cannot append null value
Throwable occurred: java.lang.NullPointerException
at org.apache.coyote.ajp.AjpMessage.appendString(AjpMessage.java:242)
at org.apache.coyote.ajp.AjpMessage.appendBytes(AjpMessage.java:178)
at org.apache.coyote.ajp.AjpAprProcessor.prepareResponse(AjpAprProcessor.java:962)
at org.apache.coyote.ajp.AjpAprProcessor.action(AjpAprProcessor.java:486)
at org.apache.coyote.Response.action(Response.java:183)
at org.apache.coyote.Response.sendHeaders(Response.java:379)
at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:305)
at org.apache.catalina.connector.OutputBuffer.close(OutputBuffer.java:273)
at org.apache.catalina.connector.CoyoteOutputStream.close(CoyoteOutputStream.java:104)
at com.jcraft.jzlib.DeflaterOutputStream.close(DeflaterOutputStream.java:122)
at org.kohsuke.stapler.compression.FilterServletOutputStream.close(FilterServletOutputStream.java:36)
at java.io.FilterOutputStream.close(FilterOutputStream.java:154)
at sun.nio.cs.StreamEncoder$CharsetSE.implClose(StreamEncoder.java:454)
at sun.nio.cs.StreamEncoder.close(StreamEncoder.java:183)
at java.io.OutputStreamWriter.close(OutputStreamWriter.java:282)
at java.io.BufferedWriter.close(BufferedWriter.java:259)
at org.dom4j.io.XMLWriter.close(XMLWriter.java:286)
at org.kohsuke.stapler.jelly.HTMLWriterOutput.close(HTMLWriterOutput.java:70)
at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:56)
at org.kohsuke.stapler.jelly.JellyClassTearOff.serveIndexJelly(JellyClassTearOff.java:112)
at org.kohsuke.stapler.jelly.JellyFacet.handleIndexRequest(JellyFacet.java:127)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:716)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:841)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:794)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:841)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:630)
at org.kohsuke.stapler.Stapler.service(Stapler.java:224)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:96)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:88)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:48)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:135)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:174)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ApiTokenFilter.doFilter(ApiTokenFilter.java:64)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:46)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:438)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.ajp.AjpAprProcessor.process(AjpAprProcessor.java:421)
at org.apache.coyote.ajp.AjpAprProtocol$AjpConnectionHandler.process(AjpAprProtocol.java:378)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1509)
at java.lang.Thread.run(Thread.java:738)