i've searched the zimbra.log file after a new try of user-login and found this; perhaps this can help someone to solve my issue:
2006-02-09 18:34:22,020 INFO [http-443-Processor99] [mid=5;ip=192.168.0.75;name=user@yourdomain.local;] cache - Initializing folder and tag caches for mailbox 5
2006-02-09 18:34:22,385 INFO [Thread-263] [] im - Executing IMEvent: IMPresenceUpdateEvent: From:
user@yourdomain.local Presence: ONLINE pri=1 st=null
2006-02-09 18:35:43,443 WARN [http-443-Processor98] [] StandardWrapper[/service:ProxyServlet] - Servlet.service() for servlet ProxyServlet threw exception
java.net.SocketException: Host is down
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl .java:333)
at java.net.PlainSocketImpl.connectToAddress(PlainSoc ketImpl.java:195)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.j ava:182)
at java.net.Socket.connect(Socket.java:507)
at java.net.Socket.connect(Socket.java:457)
at sun.net.NetworkClient.doConnect(NetworkClient.java :157)
at sun.net.
http://www.http.HttpClient.openServe...lient.java:365)
at sun.net.
http://www.http.HttpClient.openServe...lient.java:477)
at sun.net.www.http.HttpClient.<init>(HttpClient.java :214)
at sun.net.
www.http.HttpClient.New(HttpClient.java:287)
at sun.net.
www.http.HttpClient.New(HttpClient.java:299)
at sun.net.
http://www.protocol.http.HttpURLConn...ction.java:792)
at sun.net.
http://www.protocol.http.HttpURLConn...ction.java:744)
at sun.net.
http://www.protocol.http.HttpURLConn...ction.java:669)
at sun.net.
http://www.protocol.http.HttpURLConn...ction.java:913)
at java.net.HttpURLConnection.getResponseCode(HttpURL Connection.java:367)
at com.zimbra.cs.zimlet.ProxyServlet.fetchURLContent( ProxyServlet.java:212)
at com.zimbra.cs.zimlet.ProxyServlet.getURLContent(
Pr oxyServlet.java:252)
at com.zimbra.cs.zimlet.ProxyServlet.doProxy(ProxySer vlet.java:297)
at com.zimbra.cs.zimlet.ProxyServlet.doGet(ProxyServl et.java:268)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:689)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:152)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:802)
at org.apache.catalina.core.ApplicationFilterChain.in ternalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.do Filter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invo ke(StandardWrapperValve.java:214)
at org.apache.catalina.core.StandardContextValve.invo ke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke( StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke (ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invok e(StandardEngineValve.java:107)
at org.apache.catalina.valves.AccessLogValve.invoke(A ccessLogValve.java:526)
at org.apache.catalina.connector.CoyoteAdapter.servic e(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(H ttp11Processor.java:825)
at org.apache.coyote.http11.Http11Protocol$Http11Conn ectionHandler.processConnection(Http11Protocol.jav a:738)
at org.apache.tomcat.util.net.PoolTcpEndpoint.process Socket(PoolTcpEndpoint.java:526)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThr ead.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlR unnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:613)