Acronis True Image 2019: Office 365 backup fails with "login for the account to which the mailbox belongs is also used for another Microsoft account"

Symptoms

1. You run a backup of Microsoft Office 365 mailbox with Acronis True Image 2019
2. Backup fails with the following error: 

   Cannot back up this mailbox because the login for the account to which the mailbox belongs is also used for another Microsoft account

Cause

There are two two types of Microsoft accounts: Personal account and Work or school account. If you have a Personal account and a Work or school account tied to the same e-mail address, Office 365 mailbox systems provide incorrect replies to commands utilized for accessing data to back up.

Solution

You can use the following workaround:

1. Go to manage account page on https://account.live.com and create an alias email address, with a login different from theaccount that produces the error (click Manage your sign-in email or phone number ->

Select Add email). Set the newly created alias as primary alias.

You can find instructions on creating and managing Microsoft account aliases in this Microsoft Support article

2. In Online Dashboard, use the newly created alias to add new Microsoft Office 365 for Home backup.