Making black bread in a microwave oven
As you probably know, volkornbrot and similar breads are designed to bake at a lower temperature for a longer time.  240 C sounds way too high to me.  150-180 C should be about right, and leave the bread in for a long time (up to 3 hours), until the loaf starts to pull away from the sides of the pan.  Even after this baking process, because the bread batter is much wetter than a standard bread, you should not slice or eat the bread until it has had time to cool and "rest," about 24 hours.  If you slice it right away, the heat and moisture escape and the inside is still doughy/raw. If you wait, the moisture will have time to distribute throughout the loaf and the inside will firm up.  Just be patient!  The benefit is that the loaf should last for a very long time wrapped in plastic, especially if you put the unused portion in the fridge.