There was a problem opening the Photo Library. This is caused by not allowing access to Location data in the prompt the first time the Photo Library was opened within FTP On The Go. (Quite possibly, the prompt you saw just a moment ago.)
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 "General" item, then the "Reset" at the bottom of that list.
The "Reset Location Warnings" is the one to do.
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.