I guess the error is trying to say your mailbox was unable to find the shared object from your friend's. Can you grep for any errors from mailbox.log to see whats happening right after you share the folder?
You didn't mention how you are trying to mount the folder. You can try mounting with cli, with zmmailbox command. See this doc and look for 'Setting Up A Share - CLI'
Ajcody-User-Management-Topics - Zimbra :: Wiki .. hope that helps you Nikhil.