Ever since upgrading zimbra desktop when asked to, I haven't been able to fetch a single new email.
The "Error Reports" pseudo-folder contains a number of messages like this one:

Product name: Zimbra Desktop
Product version: 0.91
Build ID: 1338
Release type: BETA
OS Platform: Windows Vista x86 6.0
Time of event: 2008-10-08 17:08:04
Error type: UID STORE failed: UID STORE error; can't store that data
Error summary: Synchronization of folder '/Inbox' failed. Synchronization has been disabled for this folder

Failure details:
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com.zimbra.cs.mailclient.CommandFailedException: UID STORE failed: UID STORE error; can't store that data
at com.zimbra.cs.mailclient.imap.ImapRequest.checkSta tus(ImapRequest.java:94)
at com.zimbra.cs.mailclient.imap.ImapRequest.sendChec kStatus(ImapRequest.java:80)
at com.zimbra.cs.mailclient.imap.ImapConnection.uidSt ore(ImapConnection.java:399)
at com.zimbra.cs.mailclient.imap.ImapConnection.uidSt ore(ImapConnection.java:392)
at com.zimbra.cs.datasource.ImapFolderSync.updateFlag s(ImapFolderSync.java:697)
at com.zimbra.cs.datasource.ImapFolderSync.pushModifi cation(ImapFolderSync.java:317)
at com.zimbra.cs.datasource.ImapFolderSync.pushChange s(ImapFolderSync.java:268)
at com.zimbra.cs.datasource.ImapFolderSync.syncMessag es(ImapFolderSync.java:179)
at com.zimbra.cs.datasource.ImapSync.syncMessages(Ima pSync.java:217)
at com.zimbra.cs.datasource.ImapSync.syncFolders(Imap Sync.java:159)
at com.zimbra.cs.datasource.ImapSync.importData(ImapS ync.java:120)
at com.zimbra.cs.offline.YMailImport.importData(YMail Import.java:36)
at com.zimbra.cs.datasource.DataSourceManager.importD ata(DataSourceManager.java:158)
at com.zimbra.cs.mailbox.LocalMailbox.importData(Loca lMailbox.java:354)
at com.zimbra.cs.mailbox.LocalMailbox.syncAllLocalDat aSources(LocalMailbox.java:327)
at com.zimbra.cs.mailbox.LocalMailbox.sync(LocalMailb ox.java:383)
at com.zimbra.cs.mailbox.LocalMailbox.syncOnTimer(Loc alMailbox.java:307)
at com.zimbra.cs.mailbox.DesktopMailbox$1.run(Desktop Mailbox.java:259)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

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Note that I am synchronizing two yahoo accounts and one gmail account. The Gmail account doesn't report any error, but both yahoo accounts show those error reports, and no new mail showing when there is in fact new mail in the web interface.

I'm now going to purge the local store for one of those accounts and see if that helps any.