Exactly the same Problem here
We have a mix of XP sp3 vm's and win7-64bit desktops. Installed the beta as Pheonix suggested - on the win7-64 desktop - test system - the install failed to initialize. just removed and reinstalled the 7.0.1 GA release and now re syncing 4.2 GB of mail.
I made and ajustment to the VM environment which required that I reboot the XP vm's last night.
Checked that Zimbra Desktop came up. It was fine for 20 mins so I left the office.
Called at 7:30am 1 user could not use zimbra - problem as described.
An hour later another user had the same problem.
There are two users whose desktops were rebooted which do not have the problem.
The XP SP3 desktops (All VM's) are built from an image. They are precisely the same. Seems odd that this occurs "spontainiously" and is quite random.
The ONLY fix I can find is to force a rebuild. As my users have mail dating back 6 years some of them have 8 GB of mail. The rebuild takes 8 hours !
Please advise ASAP what the solution is.
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