A more universal way would be to get the thumbnail via oEmbed API.
//second false skip ACF pre-processcing
$url = get_field('videos', false, false);
//get wp_oEmed object, not a public method. new WP_oEmbed() would also be possible
$oembed = _wp_oembed_get_object();
//get provider
$provider = $oembed->get_provider($url);
//fetch oembed data as an object
$oembed_data = $oembed->fetch( $provider, $url );
$thumbnail = $oembed_data->thumbnail_url;
$iframe = $oembed_data->html;
It was a conflict with the theme, it provided datepicker and datetimepicker function and css, for tk plugins. Just deregistered the styles and scripts.
Are your dates send in the form? In firefox you can look in the DevTools under Network tab, in the post request data.
