This error occurs when there are folders within the mailbox that the mailbox user does not have full access to.

Full access permissions need to be allowed throughout the mailbox for the user.

Please contact your Email Servers administrator for assistance with mailbox permissions. 

It may be possible to set administrator level access (if you have it) by editing the project from the project (Home) page and using admistrator level access rather than using mailbox level permmissions.