Authentication fails every now and then

Every now and then, principal authentication fails with Confluence.

The setup is using Seraph SSO, and when typing credentials to Confluence login page, the login might fail. If redone, goes in ok.

Log:

2007-05-09 15:02:34,027 FATAL [TP-Processor6] [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:23)
at com.atlassian.crowd.integration.http.HttpAuthenticator.authenticate(HttpAuthenticator.java:165)
at com.atlassian.crowd.integration.seraph.CrowdAuthenticator.login(CrowdAuthenticator.java:53)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:127)