AdMob test ads no longer appear, showing No ad config error message

I’m trying to test the ads in my app, I’m using the following test ads unit :

<string name="APP_ID">ca-app-pub-3940256099942544~3347511713</string>
<string name="INTERSTITIAL_ADMOB">ca-app-pub-3940256099942544/1033173712</string>


InterstitialAd.load(activity, StringUtils.getString(R.string.INTERSTITIAL_ADMOB), adRequest, new InterstitialAdLoadCallback() {
                @Override
                public void onAdLoaded(@NonNull InterstitialAd interstitialAd) {
                    interstitialAd.show(activity);
                    interstitialAd.setFullScreenContentCallback(new FullScreenContentCallback() {
                        @Override
                        public void onAdDismissedFullScreenContent() {
                            adsCallBack.onAdClosed();
                            Log.d("TAG", "The ad was dismissed.");
                        }

                        @Override
                        public void onAdShowedFullScreenContent() {
                            Log.d("TAG", "The ad was shown.");
                        }
                    });
                }

                @Override
                public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
                    Log.d("AdMob", "==> " +loadAdError.getMessage());
                }
            });

I always get : No ad config

I’ve added the following line to my app-ads.txt

google.com, pub-3940256099942544, DIRECT, f08c47fec0942fa0

AdMob SDK :

implementation 'com.google.android.gms:play-services-ads:20.4.0'

Response Info :

   {
      "Response ID": "null",
      "Mediation Adapter Class Name": "",
      "Adapter Responses": []
    }

NOTE : The real ads are working fine, but the test one no.

Answer

Solution is updating the AdMob SDK to 20.5.0