Errors when cloning sites - images not copying

I'm attempting to clone a site but the images aren't copying over properly. I'm getting a "remote file is incorrect size" error for every single image. I tried commenting out the piece of code that causes this error (as found here: https://wordpress.org/support/topic/failed-to-import-media-remote-file-is-incorrect-size but found the equivalent bit in the cloner plugin). Doing so means that some of my images are now getting copied over; however, I am now getting the error "error getting class (attachment)" for most of the images.

Any idea why this is happening and what I can do to fix?