OAuth2.0 token strange behaviour (Invalid Credentials 401)

Question

Usually, Google OAuth2.0 mechanism is working great.

1. The user confirms permission to access Google account with selected scopes.
2. The refresh token is

Answer 1

I'm on Development environment. I had this problem too.

First I tried refreshing the credentials. No result. Then I deleted my app (since I'm still on development enviroment, that was ok, but BE CAREFUL WITH THIS ACTION if you're already using this on production), created a new one, updated the credentials JSON on the client... still, no result.

I solved it by opening on a new browser instance which wasn't logged in my Google Account (Private Browsing, since I'm on Firefox), logged on my Google Account once again, and tried using my client (which is a Web Application). I was redirected to the authorization screen as expected and after that, it worked fine for me.