User not registered in the Developer Dashboard error with app owner

[hr02]

Plan

Free/Premium

Operating System

Windows 10

My Question or Issue

I am getting a 403 error with the message that user is not registered in the Developer Dashboard when I try to access restricted endpoints (i.e. /me). This happened when I logged in as a tester for an app created by a different user (who added my correct email on their dashboard and yet I still received this). It was working for me just fine until yesterday. It works for the other testers of the app. So, I made my own app in the developer dashboard and tested the script with those credentials and received the same error. 

I tried clearing the cache and using different browsers but still the same effect. 

CLIENT ID: c2f2dafae77b4e288c61a961ced63b36

[kbatrhumbix]

I'm getting the exact same error, and Support Chat was not helpful at all. This is incredibly frustrating...