error while proxying request to target server: HTTP/1.1 503 Service Unavailable

[fricciobjlab]

Hello All,
I'm facing this error since three days;
here is the detailed condition:
om.zimbra.common.service.ServiceException: error while proxying request to target server: HTTP/1.1 503 Service Unavailable
ExceptionId:btpool0-5:1578605268361:dec75477a6be019c
Code:service.PROXY_ERROR Arg:(url, STR, "https://mail.objlab.it/service/soap/SyncRequest")
at com.zimbra.common.service.ServiceException.PROXY_ERROR(ServiceException.java:323)
at com.zimbra.common.soap.SoapHttpTransport.invoke(SoapHttpTransport.java:247)
at com.zimbra.common.soap.SoapHttpTransport.invoke(SoapHttpTransport.java:164)
at com.zimbra.common.soap.SoapTransport.invoke(SoapTransport.java:407)
at com.zimbra.common.soap.SoapTransport.invokeWithoutSession(SoapTransport.java:393)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:690)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:652)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:647)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:640)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:636)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:632)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequest(ZcsMailbox.java:628)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequestWithNotification(ZcsMailbox.java:624)
at com.zimbra.cs.mailbox.DeltaSync.sync(DeltaSync.java:103)
at com.zimbra.cs.mailbox.DeltaSync.sync(DeltaSync.java:89)
at com.zimbra.cs.mailbox.MailboxSync.sync(MailboxSync.java:192)
at com.zimbra.cs.mailbox.ZcsMailbox.sync(ZcsMailbox.java:125)
at com.zimbra.cs.service.offline.OfflineSync.handle(OfflineSync.java:55)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEngine.java:412)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:287)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:158)
at com.zimbra.soap.SoapServlet.doWork(SoapServlet.java:303)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.java:217)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
at com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:206)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:814)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:390)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:218)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:422)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.handle(RewriteHandler.java:230)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:585)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:988)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:415)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:429)

Server Zimbra Network Edition, Centos 7, 64 bit, under VMWARE

I tried even with ./zmproxyconfgen -v, but the error persist.
The point is that I cannot eve back up the mailboxes, due to the 503 error issued on every command.
Awaiting for some precious help,
BRegards,
Fabio Ricci