The folder permissions (accessed by right clicking on the folder within outlook and selecting properties then permissions) have been altered and possibly locked down to specific users. Please make sure they are the same as all other folders and have no restrictions in place.
You could also try creating a new outlook profile and then running outlook.exe /resetfolders . This restores missing folders for the default delivery location. Please check with your systems administrator before doing this.
This error relates to security restrictions or folder issues within the exchange server and is not a fault with Migration Monster.