Had this exact issue. It was because I tried the ambient mode and the TV downloads all kinds of images for this mode. Then, it can't release the memory. Een though the TV says you have enough memory, you don't. The solution for me was to reset the TV, install all my old apps, never open Ambient Mode, them I could upload images from USB.
So just to test things out I've performed a clean reset and to my surprise and complete shock, out of the box, with no user content, the installed junk on the TV takes a whooping 3GB of space leaving me only 856MB for everything?! Samsung, I'm asking you - what was the design decision and who was so stupid (you should probably fire them) to build a Frame TV, designed to handle user content like high-res pictures, the whole ambient stuff, downloaded art, etc and give it only 4GB of storage (really!!! How cheap do you get?) and on top of that, use up 78% of that for some unknown junk that I can't remove (Why can't I delete pre installed apps, does greed comes first?!). This is completely unacceptable and MUST be resolved!!! I would like to hear a prompt reply from Samsung on the acceptable resolution otherwise this junk is going back to the store and will be the last Samsung product I or my family will ever buy!!!