Error getting ads from AdMob using MoPub mediation


We are trying to implement MoPub into one of our apps and we have problems with mediation. We have added AdMob and requested banner ads and we receive the following error:

MoPubErrorCode: Third-party network failed to respond in a timely manner.
MoPub: Third-party network timed out.

We have tested the integration with Marketplace and Orders successfully. We also used a VPN to request ads from a US location to make sure we can receive ads and the problem is the same. Note that the AdMob banner id we are using is active on AdMob and receiving impressions directly from the AdMob integration.

We have double-checked the mediation integration for AdMob (we used the mediation wizard to get the required gradle entries). It looks like the problem is with the time it takes for AdMob to respond, but it does not take more than 1 or 2 seconds from the time the ad is requested to the time the error is displayed.

Does someone know why this happens and how to correct it?