Error in mail config - No ClassLoaders found for SMTPMailServerImpl

Configuring an SMTP server in Confluence against Jboss seems to work initially, and then after a day or so with the following error (blocking all access to the global administration page):

ava.lang.RuntimeException: Error in mail config: Error in mail config: Error at (4, 16: No ClassLoaders found for: com.atlassian.mail.server.impl.SMTPMailServerImpl
at com.atlassian.mail.config.ConfigLoader.(ConfigLoader.java:61)
at com.atlassian.mail.config.ConfigLoader.getServerManager(ConfigLoader.java:83)
at com.atlassian.mail.config.ConfigLoader.getServerManager(ConfigLoader.java:78)
at com.atlassian.mail.MailFactory.getServerManager(MailFactory.java:35)
at com.atlassian.confluence.admin.actions.AdministrationConsoleAction.checkEmailConfiguration(AdministrationConsoleAction.java:94)
at com.atlassian.confluence.admin.actions.AdministrationConsoleAction.doDefault(AdministrationConsoleAction.java:77)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at com.opensymphony.xwork.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:302)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor.intercept(DefaultWorkflowInterceptor.java:55)
at com.atlassian.confluence.core.ConfluenceWorkflowInterceptor.intercept(ConfluenceWorkflowInterceptor.java:33)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.atlassian.confluence.core.ConfluenceValidationInterceptor.intercept(ConfluenceValidationInterceptor.java:16)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.atlassian.confluence.security.actions.PermissionCheckInterceptor.intercept(PermissionCheckInterceptor.java:40)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.atlassian.confluence.pages.actions.PageAwareInterceptor.intercept(PageAwareInterceptor.java:73)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.atlassian.confluence.core.ConfluenceAutowireInterceptor.intercept(ConfluenceAutowireInterceptor.java:25)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.atlassian.confluence.user.actions.AuthenticatedUserInterceptor.intercept(AuthenticatedUserInterceptor.java:51)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.atlassian.confluence.util.XWorkTransactionInterceptor.intercept(XWorkTransactionInterceptor.java:133)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.DefaultActionProxy.execute(DefaultActionProxy.java:115)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.serviceAction(ServletDispatcher.java:229)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.service(ServletDispatcher.java:199)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:360)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:342)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.atlassian.confluence.util.profiling.ProfilingPageFilter.parsePage(ProfilingPageFilter.java:159)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:168)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:170)
at bucket.custom.FlushingSpringSessionInViewFilter.doFilterInternal(FlushingSpringSessionInViewFilter.java:30)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:73)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132)
at com.atlassian.confluence.util.error.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:36)
at org.mortbay.jetty.servlet.WebApplicationHandler$Chain.doFilter(WebApplicationHandler.java:334)
at org.mortbay.jetty.servlet.WebApplicationHandler.dispatch(WebApplicationHandler.java:286)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:558)
at org.mortbay.http.HttpContext.handle(HttpContext.java:1714)
at org.mortbay.jetty.servlet.WebApplicationContext.handle(WebApplicationContext.java:507)
at org.mortbay.http.HttpContext.handle(HttpContext.java:1664)
at org.mortbay.http.HttpServer.service(HttpServer.java:863)
at org.jboss.jetty.Jetty.service(Jetty.java:460)
at org.mortbay.http.HttpConnection.service(HttpConnection.java:775)
at org.mortbay.http.HttpConnection.handleNext(HttpConnection.java:939)
at org.mortbay.http.HttpConnection.handle(HttpConnection.java:792)
at org.mortbay.http.SocketListener.handleConnection(SocketListener.java:201)
at org.mortbay.util.ThreadedServer.handle(ThreadedServer.java:289)
at org.mortbay.util.ThreadPool$PoolThread.run(ThreadPool.java:455)