I found the answer for my specific case: It seemed that Dell didn't offer the full drivers for the attachable keyboard. That's why my laptop always started in tablet mode and once the keyboard got reattached the driver restarted and recognised the laptop as a laptop with a physical keyboard.
My only workaround so far is to stop that driver (which doesn't cause any other problems). Specifically it was the driver which automatically activated the digital keyboard in tablet mode. Now I only need to launch the digital keyboard manually once in a while.