It's hard to guess like that, but you seem to have some issue with autodiscover. You can either use the firstname.lastname@example.org address as a workaround, or better, make sure Autodiscover is working correctly.
To get the proper autodiscover response from Outlook, hold the CTRL key and right-click on Outlook's icon in the tray, select "Test E-Mail Auto-configuration". Enter the address of the shared mailbox (or any other mailbox for that matter), dont enter any password and clear the two Guessmart checkboxes. When propmted for credentials, make sure you enter *your* UPN/password, as you cannot login directly to a shared mailbox. If the test fails, post the results from the Log tab here so we take a proper look.
Seems it was only need to use upn and password to make it work (did'nt happen before), I tested with a recently migrated shared mailbox that was giving me same issue and after providing my upn\password 2 times in worked.