Some details on how it works. There is a sort of a bug on MS side: when Veeam Backup for Office 365 requests a list of all organization mailboxes (in case 'All Mailboxes' is selected in a job), sometimes mailboxes that do not have Exchange Online license assigned in Office 365 are returned (those with the ‘SKUAssigned’ attribute set to ‘False’ or empty). That account is showed in Exchange admin center as a regular licensed account with a mailbox. However, in the main console (https://portal.office.com/adminportal/home#/users
) this user is shown as not licensed ('No product license have been assigned' is displayed in 'Product license') or doesn't have Exchange Online license (Exchange Online plan: Off). Access to such mailbox is not allowed (including admin access using impersonation). On attempt to access it, user will get an error saying: 'Error occurred. Mailbox is unavailable. Probably license is expired for this mailbox'. So there is no way to backup such mailboxes.
The easiest workaround is to assign Exchange Online license for that mailbox, click 'Save', and then revoke the license switching it back to 'Off'. In this case, account will become Unlicensed and will not be returned in our request for organization mailboxes, resulting in successful backup job.
How the product handles these mailboxes: in GA we silently skipped them, but we had several requests to change this behavior, since we're literally hiding a problem from an admin. So in Update 1 the job will finish with a warning giving admin a chance to react. This doesn’t affect other mailboxes that will be successfully backed up and restore point for them will be successfully created. Admin will get an email notification that some mailboxes he/she intended to backup were not processed because of a licensing issue and can decide whether he/she wants to backup them during a retry (so he needs to provide a valid license for them) or not (in this case he needs to assign and revoke license as described above).