It seems that it is a URL that does not exists.
It could be better to have a 404 instead of a 500
2016-02-01 00:38:14,593 ERROR [sitemap.handled-errors] (http-bio-8080-exec-17;/generate/skins/facdroit/resources) Sitemap: error when calling sub-sitemap at <map:mount> - resource://org/ametys/runtime/kernel/sitemap.xmap:204:109 org.apache.cocoon.ProcessingException: Sitemap: error when calling sub-sitemap at <map:mount> - resource://org/ametys/runtime/kernel/sitemap.xmap:204:109 at org.apache.cocoon.ProcessingException.throwLocated(ProcessingException.java:113) at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:122) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47) at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.handleCocoonRedirect(ConcreteTreeProcessor.java:300) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.access$000(ConcreteTreeProcessor.java:49) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor$TreeProcessorRedirector.cocoonRedirect(ConcreteTreeProcessor.java:348) at org.apache.cocoon.environment.ForwardRedirector.redirect(ForwardRedirector.java:60) at org.apache.cocoon.components.treeprocessor.sitemap.RedirectToURINode.invoke(RedirectToURINode.java:70) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47) at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178) at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254) at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:118) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47) at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178) at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254) at org.apache.cocoon.Cocoon.process(Cocoon.java:699) at org.apache.cocoon.servlet.CocoonServlet.service(CocoonServlet.java:1159) at org.ametys.runtime.servlet.RuntimeServlet._doService(RuntimeServlet.java:135) at org.ametys.runtime.servlet.RuntimeServlet.service(RuntimeServlet.java:92) at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611) at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:314) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745) Caused by: java.lang.NullPointerException at org.ametys.web.URIPrefixHandler.getAbsoluteUriPrefix(URIPrefixHandler.java:153) at org.ametys.web.URIPrefixInputModule.getAttribute(URIPrefixInputModule.java:106) at org.apache.cocoon.components.treeprocessor.variables.PreparedVariableResolver.processModule(PreparedVariableResolver.java:272) at org.apache.cocoon.components.treeprocessor.variables.PreparedVariableResolver.resolve(PreparedVariableResolver.java:209) at org.apache.cocoon.components.treeprocessor.variables.VariableResolver.buildParameters(VariableResolver.java:107) at org.apache.cocoon.components.treeprocessor.sitemap.ActTypeNode.invoke(ActTypeNode.java:95) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47) at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143) at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69) at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236) at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178) at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254) at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:118) ... 56 more