I have exactly the same issue
Zimbra version 1.9.0.1
using WinXP-SP3, all English settings: I have uninstalled and reinstalled it several times, changed partition etc etc, as I downloaded Zimbra just last night I assume I have the latest release ?? using dialup modem that took a while
The problem seems to be PC & MAC all OS's, seems certainly a software issue not a local machine one?? could it be that it wants to be exclusive to any other email programme?? will taking Thunderbird off fix it??