Hi Jan, sometimes that gap at the top of the loaf, right below the crust, can be a sign that the dough was a little over-proofed going into the oven. What might be happening is the internal structure of the loaf is beginning to breakdown a bit, which means the gluten isn't quite as able to hold those gas bubbles in place, and they tend to rise to the surface of the loaf and get trapped beneath the crust. Since this batter is so warm coming off the mixer, extending the rise time before baking might well lead to slightly over-proofed dough. Another possible cause would be using a different brand of all-purpose flour, since other brands of AP flour don't tend to be as high in protein as our flour, they aren't going to develop as much gluten and won't absorb quite as much liquid. In a wet batter like this you might not initially notice much difference when using a different type of flour, but ultimately the batter will be weaker and less able to hold those gase bubbles in place when the bread hits the hot oven. One more possible cause would be if when you put the batter into the pan you didn't flatten out the surface well enough, and an air pocket remained that enlarged during baking. If you notice any large bubbles right below the surface before you put the bread in the oven, it's okay to pop them. 

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