How to find the reason if the playback does not work
The browser informs the player of all playback errors.
The message not found in the player means that the video is not available.
A decode error means that the video / audio file is corrupted or the browser does not support the codec. For example the old Firefox may not play MP4 with H.264.
The network message indicates that a some kind of network error occurred which prevented the media from being successfully fetched, despite having previously been available.
To get more information try to follow the process of downloading the media file in the browser. For example in Chrome you can open the Developer Tools and find in the Network tab a request for a media file. The Status column will indicate the error code, for example filed or 404
You can get more information if you click on this problem request.
For debugging on Android mobile devices please use a remote debugging.