If your video work on Debug mode, but on Release you see only black screen, please, check the link to your video. If you use 'http' protocol there, you will need to add next string to your AndroidManifest.xml file. Details here
<application
...
android:usesCleartextTraffic="true"
>
Devices have a maximum of simulataneous possible playback. It means you have reach this limit. Exoplayer returns: 'Unable to instantiate decoder'
known issue: This issue happen really often in debug mode.
Here are the steps to consider before opening a ticket in issue tracker
Ensure you can download to manifest / content file with a browser for exemple
Usually clear playback can be read with all Video player. Then you should ensure content can be played without any issue with another player (VideoLan/VLC is a good reference implementation)
If content is protected with an access token or any other http header, ensure you can access to you data with a wget call or a rest client app. You need to provide all needed access token / authentication parameters.
You need to record network trace to ensure communications with server is correct. Charles proxy is a simple and usefull tool to sniff all http/https calls. With this tool you should be able to analyze what is going on with network. You will see all access to content and DRM, audio / vido chuncks, ...
Then try to compare exchanges with previous tests you made.
You can try to open a ticket now !