In iCal with Zimbra iSync,you need to change your password on two differnent locations.

If you receive the following pop-up window you need to change your password. Follow the steps below.

First Change password in Zimbra iSync Preferences

1. Go to  the Apple icon and select System Preferences.
2. Under "Other," click on the Zimbra icon.
3. You will get a pop-up that says "To use the "Zimbra" perferences pane, System Preferences must quit and reopen."  Click OK.
   
4. Click on the Account tab and enter your new password.
   
5. Then click on the Sync tab and click on Sync now.
   
6. Close the Zimbra iSync window and launch iCal.

Second Change password on iCal Preferences

1. Launch iCal
2. Go to iCal menu
3. Select Preferences
4. Select Accounts
5. Next to Password enter new password
   
6. Then close iCal preference window
7. Close and reopen iCal