Hi Vu
I might have made considerable progress in finding the cause of the problem. In all our view controllers, we remove the DFPBannerView instance in viewDidDisappear. Unfortunately, this method isn't just called when a new view controller is pushed onto the navigation stack but when the overlay opens as well. So when the link for Safari is clicked, the DFPBannerView no longer exists...
So I think we can find a solution ourselves.
BTW: The reason to remove the DFPBannerViews is that we have a user interface with four tabs and each tab has a navigation controller that can build a stack of up to 20 view controllers, each displaying at least one banner. So we could easily have 40 or more view controllers with banner views if we didn't remove them. And 40 banner views take an incredible amount of memory and use more than 50% CPU (even on an iPhone 6). I suspect that the Javascript found in most creatives drives the memory and CPU requirements.
Thanks for your support
Manuel