The item cannot be saved because it has been modified externally

After filling in some fields (amongst one of them a facetselect from that selects an image from the gallery picker plugin) and pressing save I get the follow error in a dialog:

8f68c409-1264-421c-84d4-284923a1c0a1: the item cannot be saved because it has been modified externally.

In my terminal window I see the following:

ERROR - AbstractWorkflowPlugin - Invalid data to save
javax.jcr.InvalidItemStateException: 8f68c409-1264-421c-84d4-284923a1c0a1: the item cannot be saved because it has been modified externally.
at org.apache.jackrabbit.core.ItemImpl.getTransientStates(ItemImpl.java:333)
at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1080)
at org.hippoecm.repository.decorating.ItemDecorator.save(ItemDecorator.java:144)
at org.hippoecm.repository.impl.NodeDecorator.save(NodeDecorator.java:68)
at org.hippoecm.repository.decorating.checked.ItemDecorator.save(ItemDecorator.java:173)
at org.hippoecm.frontend.plugin.workflow.WorkflowAction.prepareSession(WorkflowAction.java:46)
at org.hippoecm.frontend.plugin.workflow.AbstractWorkflowPlugin.execute(AbstractWorkflowPlugin.java:208)
at org.hippoecm.frontend.plugin.workflow.AbstractWorkflowPlugin.access$000(AbstractWorkflowPlugin.java:52)
at org.hippoecm.frontend.plugin.workflow.AbstractWorkflowPlugin$2.onClick(AbstractWorkflowPlugin.java:143)
at org.apache.wicket.ajax.markup.html.AjaxLink$1.onEvent(AjaxLink.java:65)
at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:163)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:297)
at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:100)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:91)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1175)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1252)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1353)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:493)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:355)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:200)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:726)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:206)
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:324)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)