Couldnbt activate the next task in workflow

I’ve met a very strange question when I’am running the workflow: after I completed the previous task, I could not activate the next one. And I can’t find any tasks in the queue too, could anyone tell me the possible reasons? There are some exceptions were throwed to the console, but I’m now sure if it is concern about the question:

java.rmi.ConnectException: Connection refused to host: 192.168.1.220; nested exception is:
java.net.ConnectException: Connection refused: connect
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:567)
at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:185)
at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:171)
at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:101)
at com.wm.zeus.pd.authsrv.server.AuthSessionImpl_Stub.getRolesForUser(Un
known Source)
at sun.reflect.GeneratedMethodAccessor132.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at com.wm.zeus.pd.util.RMISession.invoke0(RMISession.java:286)
at com.wm.zeus.pd.util.RMISession.invoke(RMISession.java:247)
at com.wm.zeus.pd.authsrv.client.AuthSessionClient.invoke2(AuthSessionClient.java:309)
at com.wm.zeus.pd.authsrv.client.AuthSessionClient.getRolesForUser(AuthS
essionClient.java:617)
at com.wm.zeus.pd.authsrv.UserRoleService.rolesForUser(UserRoleService.java:141)
at com.wm.zeus.pd.pdclient.servlet.PDServletSession.getRoles(PDServletSession.java:543)
at com.wm.zeus.pd.pdclient.servlet.HtmlTaskVMProxy.initClient(HtmlTaskVMProxy.java:268)
at com.wm.zeus.pd.pdclient.servlet.HtmlTaskVMProxy.<init>(HtmlTaskVMProxy.java:246)
at com.wm.zeus.pd.pdclient.servlet.TaskController.startTask(TaskController.java:220)
at com.wm.zeus.pd.pdclient.servlet.PDServletSession.service(PDServletSession.java:317)
at com.wm.zeus.pd.pdclient.servlet.PDClientServlet.service(PDClientServlet.java:175)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:103)
at com.caucho.server.http.FilterChainServlet.doFilter(FilterChainServlet.java:96)
at com.caucho.server.http.Invocation.service(Invocation.java:315)
at com.caucho.server.http.CacheInvocation.service(CacheInvocation.java:135)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:246)
at com.caucho.server.http.HttpRequest.handleConnection(HttpRequest.java:164)
at com.caucho.server.TcpConnection.run(TcpConnection.java:139)
at java.lang.Thread.run(Thread.java:536)
Caused by: java.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:305)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:171)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:158)
at java.net.Socket.connect(Socket.java:434)
at java.net.Socket.connect(Socket.java:384)
at java.net.Socket.<init>(Socket.java:291)
at java.net.Socket.<init>(Socket.java:119)
at sun.rmi.transport.proxy.RMIDirectSocketFactory.createSocket(RMIDirect
SocketFactory.java:22)
at sun.rmi.transport.proxy.RMIMasterSocketFactory.createSocket(RMI