Karl This is due to the fact that there are no folder_preview_default:_preview.jpg images in the current catalog, and if there is a _preview.jpg, there will be no error.
My approach is to generate a video thumbnail and copy the image into the directory at the same time.
$videopreview = substr($path,0,strrpos($path,"/")).' /_preview.jpg';
if(!file_exists($videopreview )){
@copy($cache, $videopreview );
}
and exclude preview.jpg settings.
files_exclude:'/^preview.jpg$/i',