"Image data does not exist. Please re-upload the image."

Hello – at my wits end here.

For some bizarre reason, images with a height greater than 650 pixels will not upload properly. I get the message “Image data does not exist. Please re-upload the image.”
I have turned plugins off and saved all images as jpegs in Photoshop. Horizontal images and images with a height of less than 650 work perfectly.

I am totally puzzled so any and all help would be greatly appreciated!