Sorry for the late reply! As far as I know, mobile should work, but I haven't been able to test it on iOS.
It's likely that EF is looking for the screenshot in the wrong place, and is getting hung up waiting for it to appear. Could you try unchecking "Include screenshot" on the Feedback Form prefab? (Double check that it's also unchecked in your scene(s))
I've also edited the title of this post for my own reference (: