There is an update again. I have completely removed the water. Then scooped out further with a shot glass and absorbed and dried it with old towels. I forgot to take a photo. I will make up for it. I found the control unit. It was unplugged. So I powered it up and watched what happened. The device starts opening the flap and then closes it again. It does this 3-4 times. You can hear the motor and the flap when it closes. After that, the device gives an alarm, as if there is a backlog. I will post a photo of the control unit here. While the flap was opening and closing, I also opened the manual lever. You don't hear any water that might have accumulated inside. I suspect either the pipe inside is completely full of water (clogged), so I don't hear any water running and the sensor signals that the water is not flowing outwards and locks the flap. But it can also be that the flap is locked because the sensor is defective. But then I would expect to hear a little water splashing when I turn on the water. My question to you is, what else could it possibly be that I am not considering here. Does anyone have such a flap and know how the device behaves after initialization. Many thanks in advance!