Hi Torsten,
I'm not sure how, but the issue seems to occur only with your current template.
When I switched the site template to protostar, the video preview is displaying properly(http://take.ms/zWG4T).
Perhaps you can consult your template provider to see if something in the template is affecting the video display?

Nach folgender Änderung funktionierte es
Template "sandal 1.5.0"

/* Damit hochgeladenes Video funktioniert */



	.es-video-player {

		 

		.es-viewport {

		

			> div {

				width: 100%;

				height: 100%;

			}

			

			.plyr__video-wrapper {

				display: inline-block;

				width: 100%;

				height: 100%;

			}

		}

	}

Glad to hear that your issue has been resolved now.

As a gentle reminder, kindly start a new thread if you have any other issue in the future so it will be easier for us to manage your inquiry. I will lock and mark this thread as resolved.