The video player no longer works on safari

We're having the same issue, have not been able to find a solution. Do you have any information on the site, what kind of web server it is running? Trying to narrow down what's causing us issues.

An early resolution of problem: if I delete the soundtrack, the video plays well

Just want to connect the topics on this problem. Here is a thread, that helps to narrow it down:
https://forums.developer.apple.com/thread/51343

(h.264 MP4 Playable on iOS 9 No Longer Playable on iOS 10)

Use the tag is like this.

<video preload="none" oncontextmenu="return false" controls>

<source src="url.mp4" type="video/mp4">

</video>

Yes, we use this tag, with different attributes (preload="auto", no controls or oncontextmenu), but I tried it with exactly yours and it still doesn't work.

The source (src="url.mp4") is an absolute path in our case, like this:
src="https://our-domain.com/protected-directory/subdirectory1/subdirectory2/filename.mp4"