In addition with @macmanx answer, You may enable Debug in wp-config.php file. Because it will help you to read the actual error.
There are plenty of ways to style your videos thumbnail. I will suggest to go with plugin as you dont need to work with coding. These video thumbnail WordPress plugins can help:
https://ww.wp.xz.cn/plugins/video-thumbnails/
https://ww.wp.xz.cn/plugins/video-embed-thumbnail-generator/
https://ww.wp.xz.cn/plugins/wp-video-posts/
https://codecanyon.net/item/auto-post-thumbnail-pro/4322624 (premium)
https://ww.wp.xz.cn/plugins/video-thumbnails/
I hope you will resolve your problem using any of those plugin.
