Clay/loam is a natural stuff. This means it does not behave exactly like you would expect it from standardized mixtures like store-bought fire clay.

So, after the first big fire in the oven, many cracks and separations formed and needed to be repaired. I prepared a good amount of very rich repair clay with lots of straw/hay/sand in it:

After these fixes the oven was left to dry/cure for a few days. It hasn't cracked much more after those fixes.