I was having the same problem and no matter what I tried it still didnt work.
Finally I managed to download the apk expansion file. What I did was the following.
*Make sure your file size is correct (file size not disk size).
*Make sure versionCode matches with your manifest's versionCode.
*Make sure you have changed the SALT bytes.
Step by step:
1- Create a new app in the developer console.
2- Obtain the LVL Licence Key and repace it in your code.
3- Sign and export your application and create a zip with your expansion file without compression (compression: "store" in win rar).
4-Upload your app to google play but upload it in the Beta Testing Section. (without a production version listed, this is very important you must not have a production version saved in draft).
5- Create a google group with the people you want to allow beta acess and under the Beta Testing tab you must add that group.
5- Now its the scary part, you must click on publish. Your app will not show in google play, it will only be shown to your testing groups, TRUST ME. It should look like this:
*Note: Make sure you don't promote the beta to production or it will be shown, also notice I don't have a production version listed.
6- Under Beta Testing tab there is a link that a you must click in order to use the beta.
After that just download the app from google play, you can just type your package name for example:
https://play.google.com/store/apps/details?id=com.mydomain.myapp
and the app will be installed along with the expansion package.
If you want to test the files download, install the app and then delete the obb file downloaded. This will start the obb download process.
Hope this helps, as it did for me!
Cheers.
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