Internal display is black

Do the screen diagnostics work ok, and with adequate brightness?   Hold the D key through powerup to initiate.

Does an external monitor display an acceptable image under Windows?

Yes I saw the colored screens after trying the diagnostics, on the internal screen with good brightness.

Also the external monitor gives a good image of the Windows.

Other things I already tried: setting the BIOS to default, to factory default, downgrading the BIOS and turning off optimus. The last step unfortunately made me unable to enter the BIOS from external monitor, or at least I can’t see it. I also tried to update the nvidia graphic driver, but it also didn’t help. Until I didn’t restart the error message disappeared from device manager, but after rebooting it was there again. An other thing: I can’t open the nvidia control panel at all. Tried fixes for it but without a result.

Remove the nVidia drivers (uninstall and reboot).  Does the display operate normally on the integrated GPU?

Is it possible, that uninstalling the driver would make the external display stop functioning aswell? I'm a bit afraid, since I need my laptop for my studies.

It will not, unless there's a hardware issue with the mainboard.  That's the process of elimination the line of questions I've asked is designed to follow.  

If the system won't run on the integrated GPU, the mainboard needs to be replaced.

I got the problem. It was hardware issue. There was a bad soldering on the motherboard.

Thanks for your help!