Whenever I try to save a configuration change (either global or for one job) I get the following (it has been happening for at least a few Hudson versions):

Status Code: 500
Exception:
Stacktrace:

java.lang.RuntimeException: Failed to instantiate class hudson.matrix.LabelAxis from

{"kind":"hudson.matrix.LabelAxis$DescriptorImpl","name":"label","stapler-class":"hudson.matrix.LabelAxis"}

at hudson.model.Descriptor.newInstance(Descriptor.java:497)
at hudson.model.Descriptor.newInstancesFromHeteroList(Descriptor.java:767)
at hudson.util.DescribableList.rebuildHetero(DescribableList.java:171)
at hudson.matrix.MatrixProject.submit(MatrixProject.java:596)
at hudson.model.Job.doConfigSubmit(Job.java:1062)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:581)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:259)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:126)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:65)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:79)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:538)
at org.kohsuke.stapler.MetaClass$7.doDispatch(MetaClass.java:219)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:538)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:454)
at org.kohsuke.stapler.Stapler.service(Stapler.java:136)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:249)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:335)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:378)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
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 org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
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:142)
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:173)
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:66)
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 winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:244)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
at java.util.ArrayList.(ArrayList.java:131)
at hudson.matrix.Axis.(Axis.java:74)
at hudson.matrix.LabelAxis.(LabelAxis.java:40)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at org.kohsuke.stapler.RequestImpl.invokeConstructor(RequestImpl.java:409)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:355)
at hudson.model.Descriptor.newInstance(Descriptor.java:488)
... 55 more

Generated by Winstone Servlet Engine v0.9.10 at Mon Sep 27 08:52:00 PDT 2010