Thanks for reporting this.

I can't jump in media files using the playback bar

Does this happen for any and every video file? Or only specific types (like ogv in the example you provided)?

I can reproduce the problem on Firefox 58, using "HTML5 WebM player". Downloading https://en.wikipedia.org/wiki/File:Curiosity%27s_Seven_Minutes_of_Terror.ogv and playing it locally with mplayer, every time I jump to a different position I get Broken file, keyframe not correctly marked.

The error appears both in .ogv (as exampled above) and .webm files such as this one: https://en.wikipedia.org/wiki/File:Schlossbergbahn.webm

These are, as far as I know, the only permitted video files in Wikimedia Commons (https://commons.wikimedia.org/wiki/Commons:File_types).

T188878 made me look at the developer tools in the web browser, and it turns out to be the same problem.
As T188878 has a slightly better description, I am closing this task as a duplicate of T188878. Further handling happens in T188878.