Some issues with full screen slider and full width slider on mobile

Hello dear Kriesi team,

working on a website, I encountered some issues which I can’t figure out myself:
I’m using embed codes from flixel.com (see below) for looping videos for the full width and full screen slider. When looking at the page on a computer, there’s a black bar below the intro video, I didn’t have that problem with self hosted videos. It would be perfect, if that area would just be filled with the video entirely.
The bigger problem is on mobile phones with all sliders on the website, no matter if full screen or full width. My iPhone SE and other devices show a white box and additionally a white, bigger frame below the looping videos. And that shouldn’t be there but I have no idea where it comes from. When looking at the website on the computer with the responsive mobile design preview, there’s nothing. Can you help me?

Thank you very much in advance,
best regards.