Office 365 synchronization fails with the following error message:
"Username (login) can't be null"
The cause is a software-related issue #APSA-18908: "[O365] The new users created in MOP are not synced to the OSA".
The issue has been fixed in the Office 365 package version 17.1.2.
Please contact your Technical Account Manager or Pooled Team Associates(email@example.com) to schedule Office 365 upgrade.