Steps to reproduce:
1. go to: http://hudson/hudson/computer/node/api/xml?depth=1
2. say hello to:

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: org.kohsuke.stapler.export.NotExportableException: class hudson.model.queue.WorkUnit doesn't have @ExportedBean
org.kohsuke.stapler.Stapler.invoke(Stapler.java:572)
org.kohsuke.stapler.MetaClass$5.doDispatch(MetaClass.java:181)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.MetaClass$13.dispatch(MetaClass.java:359)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.MetaClass$5.doDispatch(MetaClass.java:181)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:453)
org.kohsuke.stapler.Stapler.service(Stapler.java:135)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:420)
org.jvnet.hudson.plugins.monitoring.HudsonMonitoringFilter.doFilter(HudsonMonitoringFilter.java:43)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:51)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.audit_trail.AuditTrailFilter.doFilter(AuditTrailFilter.java:64)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)

root cause

org.kohsuke.stapler.export.NotExportableException: class hudson.model.queue.WorkUnit doesn't have @ExportedBean
org.kohsuke.stapler.export.Model.<init>(Model.java:46)
org.kohsuke.stapler.export.ModelBuilder.get(ModelBuilder.java:25)
org.kohsuke.stapler.export.Property.writeValue(Property.java:181)
org.kohsuke.stapler.export.Property.writeValue(Property.java:106)
org.kohsuke.stapler.export.Property.writeTo(Property.java:83)
org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:156)
org.kohsuke.stapler.export.Property.writeValue(Property.java:188)
org.kohsuke.stapler.export.Property.writeValue(Property.java:151)
org.kohsuke.stapler.export.Property.writeValue(Property.java:106)
org.kohsuke.stapler.export.Property.writeTo(Property.java:83)
org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:156)
org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:153)
org.kohsuke.stapler.export.Model.writeTo(Model.java:129)
org.kohsuke.stapler.ResponseImpl.serveExposedBean(ResponseImpl.java:176)
hudson.model.Api.doXml(Api.java:87)
sun.reflect.GeneratedMethodAccessor1606.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:259)
org.kohsuke.stapler.Function.bindAndInvoke(Function.java:126)
org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:65)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:79)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.MetaClass$5.doDispatch(MetaClass.java:181)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.MetaClass$13.dispatch(MetaClass.java:359)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.MetaClass$5.doDispatch(MetaClass.java:181)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:537)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:453)
org.kohsuke.stapler.Stapler.service(Stapler.java:135)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:420)
org.jvnet.hudson.plugins.monitoring.HudsonMonitoringFilter.doFilter(HudsonMonitoringFilter.java:43)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:51)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.audit_trail.AuditTrailFilter.doFilter(AuditTrailFilter.java:64)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.
Apache Tomcat/6.0.29