title: Firebase Admin failed to fetch a valid Google OAuth2 access token - unable to verify certificate
body: <p>Any help appreciated.</p>
  
  <p>Currently developing on localhost.<br>Date and time are in sync.<br>
  Latest firebase-admin installed.</p>
  
  <p>This was working absolutely fine one day and not the next. (No changes).</p>
  
  <p>Terminal output:<br>
  <code>@firebase/database: FIREBASE WARNING: {"code":"app/invalid-credential","message":"Credential implementation provided to initializeApp() via the \"credential\" property failed to fetch a valid Google OAuth2 access token with the following error: \"unable to verify the first certificate\"."}</code></p>
  
  <p>This is the begining of the <code>app.js</code>:<br></p>
  
  <pre><code>//requires
  var radius = require("radius");
  var dgram = require("dgram");
  var admin = require("firebase-admin");    
  
  //for CHAP hashing
  var crypto = require("crypto");
  
  //Load additional radius dictionaries
  radius.add_dictionary('./radius_dictionaries');
  
  //init firebase
  var serviceAccount = require("./serviceKey.json");
  admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: "https://&lt;database&gt;.firebaseio.com/"
  });
  //init db object
  var db = admin.database();
  </code></pre>
  
  <p>This app does not present an <em>HTTPS</em> webpage, or and <em>HTTPS</em> end point for http requests. Simply connects to the Firebase database and reads data.</p>