We heard about this issue already. It is connected with server configuration.. Folder files/ have limit (number of files or size these files). Contact with server admin and he should help You..
Quick dirty hack: put half of those images into a subfolder and then edit the db files to point to those paths (for example instead of path "product_pic1.jpg" you would have "xxx/product_pic1.jpg" -- the actual file path to half of those pics would be files/xxx/).
I do not know, it was just a suggestion (about 3.1).
change path -> I do it via a custom script that erases the products and categories that have their number (ID) over 100, then generate a new ones from what is basically an excel file dump. I do not know how you managed to put 280 categories and 800 products into quick cart, but if you did it by hand... well.. my condolences.
I had this problem with images and this helped me to solve it. Better upgrade it first, that's the clearer solution.