Is it okay to use a microwave in convection mode without the turntable

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.

This is because you are using a microwave. In theory, if you could turn off the microwaves in the microwave oven, you could use the convection function to bake things. In practice, we have had several questions which indicate that this is not how convection microwaves work. They keep nuking your dough, making it inedible. In your case, they are cooking all the moisture locally, resulting in mini-steam-explosions which create blisters, and a dried out, hard and crispy dough.

I am sorry, but there is no way you can bake with a microwave, not short pastry, and not other things. One exception might be quickly eaten "microwave cupcakes" which don't have much flour to start with and are baked for a very short time. I have seen people claim that they work, but never actually tried them.

For baking, you need a non-microwaving oven. The cheapest solution is a toaster oven with a 30x30 cm inner size, they are somewhat trickier to use than a big oven, but good enough for a small budget.