Login fails with server error - XHR format incorrect

[holmesadrianh]

I have the same project on 2 different sites. One is demo and one is live. The live site I am unable to login as it fails with a server error:

Here is what I noticed to be different between the websites based on the XHR being logged in dev tools:

Working demo site:

Broken live site:

To be more specific, notice the query string.

This is the return json:
{"error":{"code":202,"message":"Invalid field \"roles\""}}

I assume this is because for the working demo site, the field names are fields[0], fields[1], fields[2] and has roles.* whereas the broken site has different characters like fields%5B0%5D, fields%5B1%5D, fields%5B2%5D and has roles.%2A

There is clearly something wrong about the live site. The formatting of the query is incorrect. Where is this formatting being done and how do I debug what the issue is?

Any assistance is much appreciated. I am hoping to resolve this soon. I am struggling and am stuck on this one issue.