Bamboo 4 doesn't build when Github webhook is triggered

When a plan is configured to use Github (with Git Repository) and has build plan "Repository triggers the build when changes are committed" it does not trigger a build when using the Github Bamboo webhook.

Instead, I get the following error message in bamboo.log:

2012-04-10 23:06:11,678 INFO [qtp13170424-30056] [AccessLogFilter] aaron GET http://bamboo.myserver.com:8085/ajax/planStatusHistoryNeighbouringSummaries.action?planKey=MYPROJECT-MYPLAN&buildNumber=null 147261kb
2012-04-10 23:06:13,826 INFO [qtp13170424-8] [AccessLogFilter] 83.240.48.196 GET http://bamboo.myserver.com:8085/ajax/viewPlanUpdates.action?sinceSystemTime=1334099153623 143346kb
2012-04-10 23:06:22,470 INFO [qtp13170424-12] [AccessLogFilter] 50.57.128.197 POST http://bamboo.myserver.com:8085/api/rest/login.action 142732kb
2012-04-10 23:06:22,702 INFO [qtp13170424-11] [AccessLogFilter] 50.57.128.197 POST http://bamboo.myserver.com:8085/api/rest/executeBuild.action 142226kb
2012-04-10 23:06:22,705 ERROR [qtp13170424-11] [DispatcherUtils] Could not find action
There is no Action mapped for namespace /api/rest and action name executeBuild. Check if there is such an action name with such namespace defined in the xwork.xml and also if such an action class exists. Check also the log to see if the action class is successfully loaded. - [unknown location]
at com.opensymphony.xwork.DefaultActionProxy.<init>(DefaultActionProxy.java:75)
at com.opensymphony.xwork.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:46)
at com.opensymphony.webwork.dispatcher.DispatcherUtils.serviceAction(DispatcherUtils.java:265)
at com.opensymphony.webwork.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:202)
at com.atlassian.bamboo.ww2.BambooFilterDispatcher.doFilter(BambooFilterDispatcher.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:25)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.opensymphony.webwork.dispatcher.ActionContextCleanUp.doFilter(ActionContextCleanUp.java:88)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.bamboo.filter.BambooProfilingFilter.doFilter(BambooProfilingFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.bamboo.filter.AccessLogFilter.doFilter(AccessLogFilter.java:66)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:265)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at com.atlassian.bamboo.filter.SeraphLoginFilter.doFilter(SeraphLoginFilter.java:71)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:149)
at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:98)
at com.atlassian.bamboo.filter.BambooAcegiProxyFilter.doFilter(BambooAcegiProxyFilter.java:25)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.bamboo.filter.LicenseFilter.doFilter(LicenseFilter.java:73)
at com.atlassian.core.filters.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:31)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.johnson.filters.AbstractJohnsonFilter.doFilter(AbstractJohnsonFilter.java:71)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:211)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.seraph.filter.BaseLoginFilter.doFilter(BaseLoginFilter.java:150)
at com.atlassian.seraph.filter.BambooLoginFilter.doFilter(BambooLoginFilter.java:34)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
at com.atlassian.oauth.serviceprovider.internal.servlet.OAuthFilter.doFilter(OAuthFilter.java:71)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
at com.atlassian.security.auth.trustedapps.filter.TrustedApplicationsFilter.doFilter(TrustedApplicationsFilter.java:98)
at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:170)
at com.atlassian.bamboo.persistence.BambooSessionInViewFilter.doFilterInternal(BambooSessionInViewFilter.java:31)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:75)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.planetj.servlet.filter.compression.CompressingFilter.handleDoFilter(CompressingFilter.java:203)
at com.planetj.servlet.filter.compression.CompressingFilter.doFilter(CompressingFilter.java:174)
at com.atlassian.bamboo.filter.CompressingFilter.doFilter(CompressingFilter.java:71)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.bamboo.filter.RequestCacheThreadLocalFilter.doFilter(RequestCacheThreadLocalFilter.java:31)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:96)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:75)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:473)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:516)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:226)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:921)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:403)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:184)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:856)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:114)
at org.eclipse.jetty.server.Server.handle(Server.java:352)
at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.java:596)
at org.eclipse.jetty.server.HttpConnection$RequestHandler.content(HttpConnection.java:1069)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:805)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:212)
at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:426)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:510)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.access$000(SelectChannelEndPoint.java:34)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:40)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:450)
at java.lang.Thread.run(Thread.java:636)
2012-04-10 23:06:22,886 INFO [qtp13170424-12] [AccessLogFilter] 50.57.128.197 POST http://bamboo.myserver.com:8085/api/rest/logout.action 141581kb
2012-04-10 23:06:25,943 INFO [qtp13170424-8] [AccessLogFilter] 83.240.48.196 GET http://bamboo.myserver.com:8085/rest/api/latest/server?_=1334099184807 141104kb