I searched but cound't find any solution yet.

If embedding an mp4 file using the responsive-template, the video doesnt fill the player window completly, but appears smaller, so showing a black border.

When changing the browser window width smaller, the video file dimensions changes differently as the black player window dimensions.

I also tried it in a clean Joomla to rule out jquery or other conflicts

here are some pics:









Here is the code used:
{mp4remote}eikboomgmbh.de/images/video/Imagefilm_Eikboom_50_1-480.mp4{/mp4remote}

and here a live preview:
support.maxign.de/index.php/testvideo

Is it a bug? or something I need to change in settings or so?

Even if it is a bug... I need it to run on a live site asap nicely.. is there a kind of workaround? need to change the video file format or so?

First of all,

Thank you for reporting this, we will look into it.
Now as a quick fix you can use the following:

Paste this snippet in your template's stylesheet.