The error message from Volley is very strange – the response code should be 200. I’m not familiar with Android development, but I did a little bit of Googling and I see several threads related to the same error message (none so far that offer a clear fix).
In order to troubleshoot this much further, we’ll need a copy of the full request and response. I’m sure there are Android tools that would let you do this, or you could put a proxy (like mitmproxy or Charles) on an upstream host and capture the traffic from there. The raw request and response should tell us what’s happening. That’s probably something you want to provide in the support ticket, or at least in a DM.
As for the two responses to one request in JS, I’ve never heard of such a thing either! If you can duplicate this, please provide the code so we can debug it here ourselves.
I’m sorry for all the trouble you’re having getting API access going. We will get to the bottom of it.