Symptoms
Office 365 task fails with error:
Method add_user calling failed. Reason:The value in the parameter FederatedUser.SourceAnchor is null or empty
Cause
Software-related issue APSA-4408: "User creation and 2-way sync fails for the O365 users in federated domains" fixed in scope of Office365 4.11.
Resolution
Please, update corresponding Office365 instance to version 4.11 or higher.