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

Maybe this is helpful to someone:

I had a similar issue using the JavaScript Google-API client for Calendar API. At random times it would work but mostly I got the same error. Adding scopes, testing key, nothing helped. After a few hours I found this solution, no idea why it works but it solved the issue for me:

gapi.client.init({
    'apiKey': API_KEY,  <-- DOESN'T WORK
    'clientId': CLIENT_ID,
    'discoveryDocs': DISCOVERY_URLS,
    'scope': SCOPE
}).then(function() {
    // gapi.client.setApiKey(API_KEY); <-- ADD THIS
})