「wp-admin/includes/image-edit.php:270行」あたりに「while (@ob_end_clean());」を追加します。

		/**
		 * Filters the WP_Image_Editor instance for the image to be streamed to the browser.
		 *
		 * @since 3.5.0
		 *
		 * @param WP_Image_Editor $image         The image editor instance.
		 * @param int             $attachment_id The attachment post ID.
		 */
		$image = apply_filters( 'image_editor_save_pre', $image, $attachment_id );
    
    while (@ob_end_clean()); //この一行を追加
		
    if ( is_wp_error( $image->stream( $mime_type ) ) ) {
			return false;
		}