I solved this by giving the local server Administrator user full control over the user's object in Active Directory - Users and Computers.
In order to do this, go to:
Active Directory - Users and Computers => domain.local => MyBusiness => Users => SBSUsers.
Then rightclick on your user, select properties => Select the "Security" tab. Then add the Administrator user to the list of users and groups, and give it full control (make sure all checkboxes are checked).
Also, when you have this problem then you need to verify if the setting "Prevent this object from accidental deletion" is unchecked on the Object tab of the same window.