ConcurrentModificationException in ClientImpl caused by toString-method on requestContext

javax.xml.ws.soap.SOAPFaultException: Fault string, and possibly fault code, not set
	at org.apache.cxf.jaxws.JaxWsClientProxy.invoke(JaxWsClientProxy.java:146)
	at $Proxy218.getFlightData(Unknown Source)
	at com.example.FlightDataAction.run(FlightDataAction.java:94)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:619)
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$EntryIterator.next(HashMap.java:834)
	at java.util.HashMap$EntryIterator.next(HashMap.java:832)
	at java.util.AbstractMap.toString(AbstractMap.java:485)
	at java.lang.String.valueOf(String.java:2826)
	at java.lang.StringBuilder.append(StringBuilder.java:115)
	at java.util.AbstractMap.toString(AbstractMap.java:490)
	at java.util.Collections$SynchronizedMap.toString(Collections.java:2026)
	at java.lang.String.valueOf(String.java:2826)
	at java.lang.StringBuilder.append(StringBuilder.java:115)
	at org.apache.cxf.endpoint.ClientImpl.processResult(ClientImpl.java:535)
	at org.apache.cxf.endpoint.ClientImpl.invoke(ClientImpl.java:486)
	at org.apache.cxf.endpoint.ClientImpl.invoke(ClientImpl.java:310)
	at org.apache.cxf.endpoint.ClientImpl.invoke(ClientImpl.java:262)
	at org.apache.cxf.frontend.ClientProxy.invokeSync(ClientProxy.java:73)
	at org.apache.cxf.jaxws.JaxWsClientProxy.invoke(JaxWsClientProxy.java:124)
	... 5 more

    protected Map<Thread, Map<String, Object>> responseContext 
        = Collections.synchronizedMap(new WeakHashMap<Thread, Map<String, Object>>());
....

    protected Object[] processResult(Message message,
                                   Exchange exchange,
                                   BindingOperationInfo oi,
                                   Map<String, Object> resContext) throws Exception {
....

        // Grab the response objects if there are any
        List resList = null;
        Message inMsg = exchange.getInMessage();
        if (inMsg != null) {
            if (null != resContext) {
                resContext.putAll(inMsg);
                if (LOG.isLoggable(Level.FINE)) {
                    LOG.fine("set responseContext to be" + responseContext);
                }
            }
            resList = inMsg.getContent(List.class);
        }
...