There was a problem opening the Photo Library. This is caused by not having Location data enabled
Because your photos can have location information encoded in the image files, you must allow access to Location data so FTP On The Go can have access to the photos.
In the main Settings app, go to the "Location Services" item, and make sure the main "Location Services" switch is turned on.
Then the next time you pick the Photo Library folder within FTP On The Go, be sure to allow access to the Location data when you're prompted.