In order to somehow limit the potential danger of this function, the program uses a "safe mirror directory". Its a kind of Recycler bin that is used to store the files that are deleted from the destination when they don't exist on te source anymore.
The program checks i this directory exists AND if the user running the backup can write to it. Often that is not the case. The safe mirror directory can be configured on Options-Advanced. But there is no guarantee that the task can write to it anyway. Bear in mind that, if running as a service, the user running the task is often NOT the same than the user operating the UI.
So what can you do to solve this problem?
The easy way:
1- locate the directory used for the safe mirror (Oprions-Advanced) or change it to some other folder.
2- Change the permissions to that folder to (again, the easies way) full permissions for all users
The not-so easy way: if you know what user is running the task, you can somehow limit the permissions to this folder.