Thanks for your reply Rofl_My_Waffle.
Yes, I am able to change the brightness using the NVIDIA Control Panel and also using the monitors OSD menu. But brightness is not the issue here, the issue is backlight. Brightness, contrast, sharpness and many more features can be adjusted trough the OSD menu, no matter what source the monitor is currently using. On the other hand backlight (intensity) can be only adjusted in Component, SCART, HDMI and TV mode. I have no idea why this is so, but have found no way of changing it when the computer is connected using VGA or DVI connection.
Also, I have tried using a DVI to HDMI cable to enable me to decrease the backlight (which is set to maximum), but the picture looks much worse than with DVI (or VGA), so now I am using a DVI cable to connect to my computer, of course without the possibility to decrease the backlight
If anyone has an idea how to solve this problem, I'm grateful for any suggestions.