Hello,
we have 2 websites deployed on Netlify. The main one is smart-guide.org, smg-cms.netlify.app is used for a blog hosted as Smart Audio Guide System | SmartGuide using a reverse proxy Netlify redirect. The site smg-cms.netlify.app uses Netlify identity for CMS (Netlify CMS), but it just throws an error “Failed to load settings from /.netlify/identity” when trying to authenticate. The problem is the identity service tries to get configuration from main site (smart-guide.org) and not from the blog one (smg-cms.netlify.app). The similar happens when I try to use Gitlab OAuth provider (even with “site_domain: smg-cms.netlify.app”). Is there any special requirement to make this work?
Hmm, that’s a tough one. Could you have folks use the CMS through the smg-cms site instead of through the “main” site? That should work better.
It was tough but I finally made it work 
I had to set these variables:
identity_url: https://smg-cms.netlify.app/.netlify/identity
gateway_url: https://smg-cms.netlify.app/.netlify/git
and comment out the netlify-identity-widget.js which currently ignores these variables (identify_url config is ignored when using identity widget to authenticate · Issue #3995 · netlify/netlify-cms · GitHub)