The signature of the delegate changed in UE4.12: a new parameter has been added as first one ( UPrimitiveComponent* OverlappedComp ). Your callback should be:
void OnBeginOverlap(UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult);