You might see the dreadful error message when trying to open a Shared mailbox mapped to a user.
Usually it’s a case of incorrect permissions being applied. The weird thing was that the user had no problems accessing the shared mailbox via OWA.
Further looking into the mailbox, I found that MAPI was not enabled. When I tried to enable MAPI from the GUI, it would not make the change.
As a result, I moved onto enabling the change via Powershell which led me to the true error.