Error when checking InBox or "Check Mail"

I upgraded from Zimbra 4.5.11 to 5.0.10 on Redhat Enterprise Linux 5.0 platform and am having the following problem:

On Firefox 3.0 on Ubuntu Linux- after I log into my Zimbra user account.. The first time I go to the list of email I get the following error box popping up:

A network service error has occurred. msg: A network service error has occurred.
method: SearchRequest
detail: HTTP response status 403


This also happens when I click on "Check Mail" button and also when I click on the "Inbox" (or any folders) in the list of folders on the left side panel.

There was no errors reported during the Zimbra upgrade from 4.5.11 to 5.0.10 and there is not much information about CSFE_SVC_ERROR in the forums..

I am still able to receive email, I simply have to click OK each time I change folders, or check email or arrive to the Mail tab from elsewhere in Zimbra.. Extremely annoying to say the least.