I have just installed Zimbra Desktop to my Windows Vista 32-bit and tried to set it up to read my IMAP mailboxes. It works partially but eventually says:

Code:
Failure communicating with remote server. Please try again later.

Debug message: Request already pending

Exception:

com.zimbra.common.service.ServiceException: system failure: Synchronization of folder '/Inbox' failed
ExceptionId:mid=3:1243686346999:5c98e6e071f3cbbe
Code:service.FAILURE
	at com.zimbra.common.service.ServiceException.FAILURE(ServiceException.java:253)
	at com.zimbra.cs.datasource.ImapSync.syncFailed(ImapSync.java:298)
	at com.zimbra.cs.datasource.ImapSync.syncMessages(ImapSync.java:260)
	at com.zimbra.cs.datasource.ImapSync.syncFolders(ImapSync.java:195)
	at com.zimbra.cs.datasource.ImapSync.importData(ImapSync.java:152)
	at com.zimbra.cs.offline.OfflineImport.importData(OfflineImport.java:88)
	at com.zimbra.cs.datasource.DataSourceManager.importData(DataSourceManager.java:154)
	at com.zimbra.cs.mailbox.LocalMailbox.importData(LocalMailbox.java:417)
	at com.zimbra.cs.mailbox.LocalMailbox.syncAllLocalDataSources(LocalMailbox.java:389)
	at com.zimbra.cs.mailbox.LocalMailbox.sync(LocalMailbox.java:431)
	at com.zimbra.cs.mailbox.LocalMailbox.syncOnTimer(LocalMailbox.java:361)
	at com.zimbra.cs.mailbox.DesktopMailbox$1.run(DesktopMailbox.java:437)
	at java.util.TimerThread.mainLoop(Unknown Source)
	at java.util.TimerThread.run(Unknown Source)
Caused by: com.zimbra.common.service.ServiceException: system failure: Error while fetching message for UID 8752
ExceptionId:mid=3:1243686346999:5c98e6e071f3cbbe
Code:service.FAILURE
	at com.zimbra.common.service.ServiceException.FAILURE(ServiceException.java:253)
	at com.zimbra.cs.datasource.ImapFolderSync.checkCanContinue(ImapFolderSync.java:1293)
	at com.zimbra.cs.datasource.ImapFolderSync.fetchMessages(ImapFolderSync.java:942)
	at com.zimbra.cs.datasource.ImapFolderSync.fetchMessages(ImapFolderSync.java:862)
	at com.zimbra.cs.datasource.ImapFolderSync.syncMessages(ImapFolderSync.java:247)
	at com.zimbra.cs.datasource.ImapSync.syncMessages(ImapSync.java:257)
	... 11 more
Caused by: java.lang.IllegalStateException: Request already pending
	at com.zimbra.cs.mailclient.imap.ImapConnection.sendRequest(ImapConnection.java:462)
	at com.zimbra.cs.mailclient.imap.ImapRequest.send(ImapRequest.java:77)
	at com.zimbra.cs.mailclient.imap.ImapRequest.sendCheckStatus(ImapRequest.java:81)
	at com.zimbra.cs.mailclient.imap.ImapConnection.uidFetch(ImapConnection.java:297)
	at com.zimbra.cs.mailclient.imap.ImapConnection.uidFetch(ImapConnection.java:343)
	at com.zimbra.cs.mailclient.imap.ImapConnection.uidFetch(ImapConnection.java:360)
	at com.zimbra.cs.datasource.ImapFolderSync.fetchMessages(ImapFolderSync.java:938)
	... 14 more

If issue persists please post description and debug information here.
Needless to say, reading the said Inbox via SSL-protected IMAP works flawlessly in Thunderbird. My "Inbox" is a pretty big e-mail set, though. The Inbox contains both e-mail messages and about 20 subfolders, some of which have subfolders. As far as I can tell, the sync has been able to fetch 626 messages in the main Inbox and all the subfolder names, but none of the subfolder contents. Additionally, it has not fetched any Inbox messages dated before Jan 6th 2007. I have a bunch of messages in there dating back to 2004.

I've used scores of different e-mail programs since 80's. Since this is my first experience with Zimbra, I'm seriously not impressed.

Besides, I tried to post this to a thread describing a similar issue but my post never appeared so I'm reposting it as a new topic. Apologies.