If I may...
Error 413 (Request Entity Too Large) indicates that the size of the data being transmitted exceeds the limit set by the server. The most common reasons include:
- Server or proxy configuration – servers like Nginx or Apache have limits (e.g., client_max_body_sizein Nginx orLimitRequestBodyin Apache) that may block requests with data exceeding the allowed size.
- Incorrect application settings – the application handling the requests may have its own limitations regarding the size of the data transmitted.
To resolve the issue, it is advisable to review the server settings and adjust them as needed for transmitting larger data packets, or limit the size of the data sent by the client. If possible try to talk with Hosting support. In the meantime, Ofi wil try to test your site on his hosting to compare results. 
BTW
Try to use smaller .mp4 file, reduce size 2x less, and test again. For example use video file that has only 3-4MB, different filename and test again. If it will work, you know where to search.