Symptoms
Task for dropping of database user fails with the error:
Cannot change owner of object 'u123456_username.aspnet_Setup_RestorePermissions' or one of its child objects because the new owner 'dbo' already has an object with the same name."
Cause
Error occurred because indeed dbo sys user already has mentioned sys objects.
Resolution
Existence of such sys objects can be checked on DB host that is mentioned in failed task. Query should be like:
select * from dbo.sysobjects where name='aspnet_Setup_RestorePermissions';
Confirm that there are such records, delete them, after that the task can be processed.