I'm trying to integrate a third party with salesforce, but I'm stuck receiving a 'down for maintenance' screen when I try to authenticate. Here's what I've tried:
1. I have a simple visual force page which does no read/writes to object records (so permissions shouldn't be an issue). I can access it when logged in as administrator.
2. A simple force.com site which points to this page. Permissions are such that I can access it publicly (once I add object record read/write that changes).
3. A connected app configured for full oauth scope (and everything else)
4. I get an oauth_token using https://test.salesforce.com/services/oauth2/token (because it's a sandbox)
5. When I access the force.com site with that oauth_token as a url param, I get a 'down for maintenance' message. Whereas before (2) I could access it completely publicly.

I have a hunch the oauth_token is failing and instead of showing an error, but I can't find any logs to support that.

Any idea how I can configure or debug a force.com site to accept connected app oauth 2 tokens?