Use external display driver for internal display

It seems you're confusing displays with GPU. Going by the specs you posted you only have ONE GPU, the Intel iGPU on the i7 CPU. So, you may have a broken internal screen but if the HDMI connected monitors work fine then the problem IS the internal screen and it's not, never was, never will be a matter of "drivers".

*In Windows I managed to use the driver for the external display for the internal one and it works* This, at first glance, reads as complete nonsense. Please explain exactly what you did in Windows and what the results were.

no the internal screen is working in Windows 10 with switched drivers. If I connect an external monitor in Windows the external Monitor has the green artifacts. Thanks for your reply by the way!

Please explain what you mean by "switched drivers", i.e., exactly what I asked before and wasn't answered.

In Windows 10 there is a tool called "Intel Graphics and Media Control Panel". It's hidden in the properties panel of the monitor. If you choose advanced settings>Display>Multiple Displays and change the settings to "Clone-Displays" the Software somehow generates a virtual display which is not present. I don't think it's meant to do that but it solves my problem in Windows. And finally if I set the new virtual monitor as primary display the green artifacts are gone and "moved" to the virtual monitor.

If I now plug in an external monitor then the artifacts are on the external screen in Windows. It was pure luck to find this solution. I don't think it's another driver if I think about it now but the external screen is not driven by the integrated GPU and I want the integrated display be driven by the CPU.

No, not a different driver nor a different GPU. What changed there (in Windows) is quite unusual but a good find nonetheless. Have you tried to toggle modes in Ubuntu as well? I mean toggling between mirrored/extended and eventually turning off one or the other? Basically the same options you have with the Intel software in Windows you have at System Settings > Screens in Ubuntu.

I tried many things including to mirror, extend or turning off the screens. I even created a virtual display with xrandr to do so without a monitor attached, but i don't get these green lines to the other monitor. Could it be possible to turn off the integrated graphics like in device-manager in Windows and let the system use only the CPU for graphics?

The system IS using the i7 integrated graphics and there's no other. Above you're almost getting there but clearly you don't understand. Please contact tech support because what you have is a problem. It may or may not be fixed by a firmware (UEFI) update but it's a good idea to try it anyway. The next step is replacing the whole unit.

yeah i know i thought that i just could get around the problem somehow without replacing. a new motherboard would cost more as the whole laptop is worth and there is no BIOS update I installed the newest firmware. I will try to contact support but I think to get it working with what I have, the Tech supporter can not help me, because he doesn't know Ubuntu. Thank you for your help anyways. I will try putting the board into the oven at low tempratures. This helped many people with broken pcb's. The problem came slowly and the artifacts grew from a point on the screen.

Tech support doesn't have to know about Ubuntu. The problem is clearly hardware and manifests itself in Windows, the preinstalled and supported OS. Your question is therefore off-topic here.

I sit in a Tesla and translated this thread with Ai: