Why does my bread ferment in the middle?
I have marked George M's answer as the accepted answer, but I thought I would post a slightly expanded answer.

I can confirm that the following ideas that  were raised in the comments did not work:


Hotter oven
Cooking longer
Cooling completely


What has worked is to wrap the bread completely in a tea towel or two until completely cool. Then, the bread stays really fresh for at least a day.

I also found that bread thus cooled can be placed in the bread bin still wrapped in the tea towel, and it doesn't rot.