This exception is seen on the client side (Confluence) seemingly randomly.

007-07-12 14:41:32,058 FATAL [http-10.10.40.96-80-Processor9] [service.soap.client.SecurityServerClient] authenticatePrincipal Unable to connect to the crowd server: null :Could not invoke service.. Nested exception is org.codehaus.xfire.fault.XFireFault: Unexpected EOF in prolog
at [row,col

{unknown-source}]: [1,0]
org.codehaus.xfire.XFireRuntimeException: Could not invoke service.. Nested exception is org.codehaus.xfire.fault.XFireFault: Unexpected EOF in prolog
at [row,col {unknown-source}

]: [1,0]
org.codehaus.xfire.fault.XFireFault: Unexpected EOF in prolog
at [row,col

{unknown-source}]: [1,0]
at org.codehaus.xfire.fault.XFireFault.createFault(XFireFault.java:89)
at org.codehaus.xfire.client.Client.onReceive(Client.java:386)
at org.codehaus.xfire.transport.http.HttpChannel.sendViaClient(HttpChannel.java:139)
at org.codehaus.xfire.transport.http.HttpChannel.send(HttpChannel.java:48)
at org.codehaus.xfire.handler.OutMessageSender.invoke(OutMessageSender.java:26)
at org.codehaus.xfire.handler.HandlerPipeline.invoke(HandlerPipeline.java:131)
at org.codehaus.xfire.client.Invocation.invoke(Invocation.java:75)
at org.codehaus.xfire.client.Client.invoke(Client.java:335)
at org.codehaus.xfire.client.XFireProxy.handleRequest(XFireProxy.java:77)
at org.codehaus.xfire.client.XFireProxy.invoke(XFireProxy.java:57)
at $Proxy42.authenticatePrincipal(Unknown Source)
at com.atlassian.crowd.integration.service.soap.client.SecurityServerClient.authenticatePrincipal(SecurityServerClient.java:34)
at com.atlassian.crowd.integration.atlassianuser.CrowdAuthenticator.authenticate(CrowdAuthenticator.java:39)
at com.atlassian.user.impl.delegation.security.authentication.DelegatingAuthenticator.authenticate(DelegatingAuthenticator.java:53)
at bucket.user.DefaultUserAccessor.authenticate(DefaultUserAccessor.java:689)
at sun.reflect.GeneratedMethodAccessor1151.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:284)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:155)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:122)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:144)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:174)
at $Proxy16.authenticate(Unknown Source)
at com.atlassian.confluence.user.ConfluenceAuthenticator.authenticate(ConfluenceAuthenticator.java:43)
at com.atlassian.seraph.auth.DefaultAuthenticator.login(DefaultAuthenticator.java:85)
at com.atlassian.confluence.user.ConfluenceAuthenticator.login(ConfluenceAuthenticator.java:26)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:127)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.confluence.util.ClusterHeaderFilter.doFilter(ClusterHeaderFilter.java:35)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:172)
at com.atlassian.spring.filter.FlushingSpringSessionInViewFilter.doFilterInternal(FlushingSpringSessionInViewFilter.java:29)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:122)
at com.atlassian.core.filters.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.confluence.util.RequestCacheThreadLocalFilter.doFilter(RequestCacheThreadLocalFilter.java:25)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.core.filters.gzip.GzipFilter.doFilter(GzipFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
Caused by: com.ctc.wstx.exc.WstxEOFException: Unexpected EOF in prolog
at [row,col {unknown-source}

]: [1,0]
at com.ctc.wstx.sr.StreamScanner.throwUnexpectedEOF(StreamScanner.java:628)
at com.ctc.wstx.sr.BasicStreamReader.handleEOF(BasicStreamReader.java:1970)
at com.ctc.wstx.sr.BasicStreamReader.nextFromProlog(BasicStreamReader.java:1883)
at com.ctc.wstx.sr.BasicStreamReader.next(BasicStreamReader.java:1026)
at org.codehaus.xfire.soap.handler.ReadHeadersHandler.invoke(ReadHeadersHandler.java:44)
at org.codehaus.xfire.handler.HandlerPipeline.invoke(HandlerPipeline.java:131)
at org.codehaus.xfire.client.Client.onReceive(Client.java:382)
... 65 more