When you optimize an image with ShortPixel Image Optimizer, you may receive the message “File Error. File could not be loaded with this ID ( Attachment ID XXX )”.
This error message usually appears when there is an entry in the Media Library, but the file does not exist or is corrupted. A quick solution to this problem is to use a plugin like Enable Media Replace to replace the missing images with new ones. ShortPixel should then automatically detect the new image and optimize it along with the thumbnails. If that doesn’t help, for all possible causes and troubleshooting methods, please read this article: Why some or all of the images cannot be optimized?