Alain,
We are experiencing the same issue and the workaround we developed is not acceptable. MS Support is pointing to the client computer when we know that is not the case since Shared Calendars broke for ALL our Mac users.
We removed all Outlook related application support files in the library for the user.
- Library > Caches
- Library > Preference Panes
- Library > Preferences
And like you, we deleted the related keychain.
Try it for one user and let me know if you find the same results.
In empathy,
--Liko
Office 365 Education Hybrid Deployment (Trial)
Exchange 2007 + Exchange 2010 Hybrid Server
Santa Rosa, California