A client was having issues with her image uploads http error and thumbnails not generating. Smaller images would generate the thumbnail just fine however larger images would upload but fail to generate thumbnails. After uploads the site was also showing an “http” error. After doing some searching I found this snippet fixed it.
add_filter( 'wp_image_editors', 'change_graphic_lib' );
function change_graphic_lib($array) {
return array( 'WP_Image_Editor_GD', 'WP_Image_Editor_Imagick' );
}
Add that to your functions.php file and see if your uploads work again.