Drag&Drop not working with Zimbra 7.1.4?

Hi everybody,
last friday we updated ZCS suite to version 7.1.4, and since that operation the drag&drop function with zimlets stopped working correctly. I tried with all these zimlets (that allow users to backup mail and contacts locally, by dragging them on the zimlet):

Email Downloader
StarXpert:Save
Email-2-doc
Folder Backup

Sometimes the operation ends without errors, and the download of the email archive starts correctly, but very often the interface freezes after the drop, and if I click in another position (e.g. on another mail) this error pops out:

Code:

---

method:    [unknown]
msg:    no such tag: 257
code:    mail.NO_SUCH_TAG
detail:    soap:Sender
trace:    btpool0-1583://serverIP/service/soap/MsgActionRequest:1327336962236:f5aa1fb8814ebc8d
request:   
Body: {
  MsgActionRequest: {
    _jsns: "urn:zimbraMail",
    action: {
      id: "257",
      op: "tag",
      tag: "257"
    }
  }
 },
Header: {
  context: {
    _jsns: "urn:zimbra",
    account: {
      _content: "yacme@server.com",
      by: "name"
    },
    authToken: "(removed)",
    session: {
      _content: 16770,
      id: 16770
    },
    userAgent: {
      name: "ZimbraWebClient - GC12 (Linux)",
      version: "7.1.4_GA_2555"
    }
  }
 }

---

This is a relative log:

Code:

---

com.zimbra.common.service.ServiceException: invalid request: missing required attribute: tag
ExceptionId:btpool0-1568://serverIP/service/soap/MsgActionRequest:1327335709080:f5aa1fb8814ebc8d
Code:service.INVALID_REQUEST
        at com.zimbra.common.service.ServiceException.INVALID_REQUEST(ServiceException.java:260)
        at com.zimbra.common.soap.Element.checkNull(Element.java:301)
        at com.zimbra.common.soap.Element.getAttributeLong(Element.java:246)
        at com.zimbra.cs.service.mail.ItemAction.handleCommon(ItemAction.java:131)
        at com.zimbra.cs.service.mail.MsgAction.handle(MsgAction.java:41)
        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$CachedChain.doFilter(ServletHandler.java:1166)
        at com.zimbra.cs.servlet.SetHeaderFilter.doFilter(SetHeaderFilter.java:79)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:81)
        at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:132)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
        at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:218)
        at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
        at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
        at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:422)
        at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
        at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
        at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
        at org.mortbay.jetty.handler.rewrite.RewriteHandler.handle(RewriteHandler.java:230)
        at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
        at org.mortbay.jetty.handler.DebugHandler.handle(DebugHandler.java:77)
        at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
        at org.mortbay.jetty.Server.handle(Server.java:326)
        at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:585)
        at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:988)
        at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
        at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
        at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:415)
        at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:429)
        at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:451)

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With version 7.1.3 there wasn't any problem.
Do you have any ideas? I reproduced the error on Firefox 9.0.1 on Linux, Explorer 9 on Win7, Chrome 15 on Linux.