You may receive an error similar to the one below when a mailbox has been selected that has no Office 365 licence associated to it.
A request is behaving in an unexpected way - please send this message to your support contact
Couldn't save Office 365 data for account mailbox@domain: Failed to read account mailbox@domain with ID
: Item not found. Status 404
To resolve this issue, either add a licence to the mailbox, so it can be used, or remove the mailbox from Office 365 and update the mailbox selection in GBM.