Clicking the X icon in Build Executor Status to cancel a hanging build yields this error:

Stack trace
javax.servlet.ServletException: java.lang.IllegalAccessException: POST is required for public void hudson.model.Executor.doStop(org.kohsuke.stapler.StaplerRequest,org.kohsuke.stapler.StaplerResponse) throws java.io.IOException,javax.servlet.ServletException
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:693)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:777)
at org.kohsuke.stapler.MetaClass$9.dispatch(MetaClass.java:299)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:684)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:777)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:201)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:684)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:777)
at org.kohsuke.stapler.MetaClass$12.dispatch(MetaClass.java:381)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:684)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:777)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:201)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:684)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:777)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:586)
at org.kohsuke.stapler.Stapler.service(Stapler.java:217)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:248)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:376)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:96)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:88)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:48)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:331)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:227)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1146)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:679)
Caused by: java.lang.IllegalAccessException: POST is required for public void hudson.model.Executor.doStop(org.kohsuke.stapler.StaplerRequest,org.kohsuke.stapler.StaplerResponse) throws java.io.IOException,javax.servlet.ServletException
at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:28)
at org.kohsuke.stapler.Function$InterceptedFunction.invoke(Function.java:398)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:160)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:95)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:111)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:684)
... 49 more