Xcode iOS App - Screen becomes unresponsive during game

I am having trouble with my quiz game - sometimes (not always) after completing a level, the screen darkens and the game becomes unresponsive. I am unable to click anywhere on the screen.

I am using Fyber mediation to run interstitial ads every 5 levels.
I can show the error log if needed.

I'd like to identify the problem and get this fixed.
Xcode, iOS 14

Many thanks, AE