Admob ad stuck in "You can skip this ad in 5 seconds"

Question

I am using Admob for my iOS application. Everything is going well, but the full-screen ad "GADInterstitial" sometimes creates a black screen with the text "You can skip this ad in 5 seconds." It remains the same with a black screen, and there is no way to miss. Any idea what could be the problem?

PS Access to Youtube is blocked here, so the problem is this video ad?

I see animated ads (possibly gifs) perfectly, but sometimes this black screen gets stuck, which requires minimizing the application and then switching back.

I just executed the code here

- (void) showAd
{
    GADInterstitial *interstitial_ = [[GADInterstitial alloc] init];
    interstitial_.adUnitID = MY_INTERSTITIAL_UNIT_ID;
    [interstitial_ loadRequest:[GADRequest request]];
}

- (void)interstitialDidReceiveAd:(GADInterstitial *)interstitial
{
    [interstitial_ presentFromRootViewController:self];
}