Does anyone know exactly, what is this circular area behind the front glas for? Is it a sensor?

This is a color temperature sensor.

Hi, Hubit. Thank you for reaching out. As mentioned in the response above, the silver right at the lower part of the camera is the Color Temperature Photo Sensor. With the help of the color temperature photo sensor, DJI Action 2 has better color temperature performance in environments with complicated lighting conditions. For example, when diving with DJI Action 2, the device is able to correct the color temperature underwater. When the shooting angle changes, the color temperature will not change drastically. Additionally, it is able to detect the strobing and thereby remove it. Should you have further concerns, please reach back here at DJI Forum. Have a nice day!

Thanks DJI Tony and fish sticks ! Does it have a negative impact on the image if this sensor is covered by a camera rig etc. and no light falls on it? Or does the white balance also work in principle without the sensor? Maybe not optimal then?

Probably best to avoid covering it. Otherwise, you can use manual white balance if you run into issues.

Hubit Posted at 1-3 02:50
Thanks DJI Tony and fish sticks ! Does it have a negative impact on the image if this sensor is covered by a camera rig etc. and no light falls on it? Or does the white balance also work in principle without the sensor? Maybe not optimal then?

This is most useful in places with poor lighting conditions. As fish sticks mentioned, we recommend you avoid blocking this sensor for optimal performance. Thank you, and have a nice day.