After upgrading Zimbra to 6.0.5 I did try again to sync Zimbra and N900 (as calendar sync was not working with Zimbra v5). N900 MfE seems to be really buggy as it has random quirks - with new and empty Zimbra account I managed to get all sorts of sync results - N900 not syncing calendar, N900 syncing calendar if creating new calendar, N900 deleteing all calendars during sync, N900 not creating new calendar during sync, N900 creating several new calendars (for each sync), N900 reporting sync success but really not syncing.

After fiddling around a bit and testing all sorts of scenarios I came to some conculsions:

1) Do create a new calendar in N900 for Zimbra sync - I never managed to get default N900 calendar to sync. In MfE configuration choose some this newly created calendar to sync - not N900! For first sync this calendar should be absolutely empty!

2) If seeing strange results from sync or sync not working properly - do reboot the N900 to have activesync daemon restarted and try again

I really cant understand why MfE in N900 is still so buggy and problematic - why Nokia did not port existing symbian and mostly working version of MfE to N900 which could have same feature set and stability in theory. Its really disapponting - I own N900 for several months now but because of poor MfE I can't use it on working days. And Im not the only one having this problem...