Dear Supporters and Developers,

We have started to having an issue with Zimbra OS edition. The problem lays in shared calendars. We managed to configure the Thunderbirds (with Lightning Add-On) and share the calendars within the users. Everything ran smoothly until suddenly one of the calendars stoped to work properly. We can not modify it remotely. Thunderbird gives us such error info:

Publishing the calendar file failed
Status code: 500: Internal Server Error
We have tried to revoke the rights, restart Zimbra etc. Nothing helped.

Zimbra Log gives this info:
Code:
[UserServlet] - Servlet.service() for servlet UserServlet threw exception
com.zimbra.common.service.ServiceException: invalid request: Changing organizer of an appointment/task to another user is not allowed: old=user1@ourserver.com, new=user1@ourserver.com
        at com.zimbra.common.service.ServiceException.INVALID_REQUEST(ServiceException.java:182)
        at com.zimbra.cs.mailbox.CalendarItem.organizerChangeCheck(CalendarItem.java:872)
        at com.zimbra.cs.mailbox.CalendarItem.processNewInviteRequestOrCancel(CalendarItem.java:626)
        at com.zimbra.cs.mailbox.CalendarItem.processNewInvite(CalendarItem.java:603)
        at com.zimbra.cs.mailbox.CalendarItem.processNewInvite(CalendarItem.java:582)
        at com.zimbra.cs.mailbox.Mailbox.addInvite(Mailbox.java:3112)
        at com.zimbra.cs.service.formatter.IcsFormatter.saveCallback(IcsFormatter.java:134)
        at com.zimbra.cs.service.formatter.Formatter$SaveOperation.callback(Formatter.java:273)
        at com.zimbra.cs.operation.Operation.schedule(Operation.java:190)
        at com.zimbra.cs.service.formatter.Formatter.save(Formatter.java:103)
        at com.zimbra.cs.service.UserServlet.doPost(UserServlet.java:438)
        at com.zimbra.cs.service.UserServlet.doPut(UserServlet.java:376)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:712)
        at com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:162)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
        at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:463)
        at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:398)
        at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:301)
        at com.zimbra.webClient.filters.SetHeaderFilter.doFilter(SetHeaderFilter.java:286)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(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.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:541)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
        at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
        at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:595)
Before this error happened we did not modified ane settings on Zimbra server. Thanks in advance for any advice!