Just checking -- sometimes File->Open Other User's Mailbox works when File->Open Other User's Folder does not.
Can you check to see if our Sharing Addin is really loaded?
In Outlook: Tools->Trust Center
Click Addins (on left)
At bottom it says Manage. Make sure COM Add-ins is in dropdown. Go...
See if Zimbra Sharing Addin is checked
You can also try to unregister and register it:
At a command prompt, go to where the addin is (probably Program Files\Common Files\System\MSMAPI\<LCID>). For English <LCID is 1033>
Then do:
regsvr32 /u /s SharingAddin.dll
regsvr32 /s SharingAddin.dll |