We have released an update for Adaxes 2018.2, 2019.1, and 2019.2, which fixes the below error. If you have a hybrid environment and are seeing this error when modifying properties of Exchange Online mailboxes, it means that Microsoft has rolled out the dual-write change to your Microsoft 365 tenant, and you need to update Adaxes to remedy the issue.
Error Message: An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Unable to update the specified properties for on-premises mastered Directory Sync objects or objects currently undergoing migration.