An unexpected error occured

Unable to get write lock on 'subscribe' method for: com.commafeed.backend.services.FeedSubscriptionService
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com.commafeed.frontend.utils.exception.DisplayException: Unable to get write lock on 'subscribe' method for: com.commafeed.backend.services.FeedSubscriptionService
	at com.commafeed.frontend.pages.GoogleImportCallbackPage.<init>(GoogleImportCallbackPage.java:94)
	at sun.reflect.GeneratedConstructorAccessor187.newInstance(Unknown Source)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
	at org.apache.wicket.session.DefaultPageFactory.newPage(DefaultPageFactory.java:170)
	at org.apache.wicket.session.DefaultPageFactory.newPage(DefaultPageFactory.java:98)
	at org.apache.wicket.DefaultMapperContext.newPageInstance(DefaultMapperContext.java:137)
	at org.apache.wicket.core.request.handler.PageProvider.resolvePageInstance(PageProvider.java:278)
	at org.apache.wicket.core.request.handler.PageProvider.getPageInstance(PageProvider.java:166)
	at org.apache.wicket.request.handler.render.PageRenderer.getPage(PageRenderer.java:78)
	at org.apache.wicket.request.handler.render.WebPageRenderer.renderPage(WebPageRenderer.java:94)
	at org.apache.wicket.request.handler.render.WebPageRenderer.respond(WebPageRenderer.java:244)
	at org.apache.wicket.core.request.handler.RenderPageRequestHandler.respond(RenderPageRequestHandler.java:165)
	at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:840)
	at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:64)
	at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:254)
	at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:211)
	at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:282)
	at org.apache.wicket.protocol.http.WicketFilter.processRequestCycle(WicketFilter.java:244)
	at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:188)
	at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:267)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
	at org.apache.tomee.catalina.OpenEJBValve.invoke(OpenEJBValve.java:45)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
	at java.lang.Thread.run(Thread.java:722)
Caused by: javax.ejb.ConcurrentAccessTimeoutException: Unable to get write lock on 'subscribe' method for: com.commafeed.backend.services.FeedSubscriptionService
	at org.apache.openejb.core.singleton.SingletonContainer.aquireLock(SingletonContainer.java:303)
	at org.apache.openejb.core.singleton.SingletonContainer._invoke(SingletonContainer.java:222)
	at org.apache.openejb.core.singleton.SingletonContainer.invoke(SingletonContainer.java:205)
	at org.apache.openejb.core.ivm.EjbObjectProxyHandler.synchronizedBusinessMethod(EjbObjectProxyHandler.java:256)
	at org.apache.openejb.core.ivm.EjbObjectProxyHandler.businessMethod(EjbObjectProxyHandler.java:251)
	at org.apache.openejb.core.ivm.EjbObjectProxyHandler._invoke(EjbObjectProxyHandler.java:85)
	at org.apache.openejb.core.ivm.BaseEjbProxyHandler.invoke(BaseEjbProxyHandler.java:279)
	at com.commafeed.backend.services.FeedSubscriptionService$LocalBeanProxy.subscribe(com/commafeed/backend/services/FeedSubscriptionService.java)
	at com.commafeed.backend.feeds.OPMLImporter.handleOutline(OPMLImporter.java:58)
	at com.commafeed.backend.feeds.OPMLImporter.importOpml(OPMLImporter.java:34)
	at com.commafeed.frontend.pages.GoogleImportCallbackPage.<init>(GoogleImportCallbackPage.java:92)
	... 37 more