Why does JBoss PicketLink throw a ResourceResolverException?

Issue

Why does JBoss PicketLink throw a ResourceResolverException?

13:21:36,455 ERROR [org.picketlink.identity.federation.web.handlers.saml2.SAML2SignatureValidationHandler] (http-/127.0.0.1:8080-1) Error validating signature:: javax.xml.crypto.dsig.XMLSignatureException: javax.xml.crypto.URIReferenceException: org.apache.xml.security.utils.resolver.ResourceResolverException: Cannot resolve element with ID ID_7261388a-ef46-40cb-b3b2-ab7c9267d367                                                                                              
        at org.apache.jcp.xml.dsig.internal.dom.DOMReference.dereference(DOMReference.java:419)
        at org.apache.jcp.xml.dsig.internal.dom.DOMReference.validate(DOMReference.java:381)
        at org.apache.jcp.xml.dsig.internal.dom.DOMXMLSignature.validate(DOMXMLSignature.java:267)
        at org.picketlink.identity.federation.core.util.XMLSignatureUtil.validate(XMLSignatureUtil.java:378) [picketlink-core-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1]                                                                  
        at org.picketlink.identity.federation.web.handlers.saml2.SAML2SignatureValidationHandler.validateSender(SAML2SignatureValidationHandler.java:95) [picketlink-core-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1]                      
        at org.picketlink.identity.federation.web.handlers.saml2.SAML2SignatureValidationHandler.handleRequestType(SAML2SignatureValidationHandler.java:66) [picketlink-core-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1]                   
        at org.picketlink.identity.federation.bindings.tomcat.idp.IDPWebBrowserSSOValve.processSAMLRequestMessage(IDPWebBrowserSSOValve.java:567) [picketlink-jbas7-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1]                            
        at org.picketlink.identity.federation.bindings.tomcat.idp.IDPWebBrowserSSOValve.invoke(IDPWebBrowserSSOValve.java:384) [picketlink-jbas7-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1]                                               
        at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.2.Final-redhat-1.jar:7.1.2.Final-redhat-1]