Acronis Backup Cloud: Office 365 backup fails with "Cannot back up the primary mailbox of an Office 365 user"

Symptoms

Office 365 backup fails with:

Error
CODE O365MailboxPrimaryMbxBackupError
MESSAGE Cannot back up the primary mailbox of an Office 365 user: 'folder: mailbox primary root: token validation failed'

Cause

Required permissions have been revoked from Acronis Agent in Office 365.

Solution

Re-register Office 365 account: