This was not an easy task. The problem was the nouveau-drivers allready installed as default.
There's a lot of how-to's, but for some reason most of the googled solutions did not work(I did an installation earlier which I do not remember how I installed correctly), but now I found another solution from a site which I do not remember but it worked very well/flawlessly.
I'm not going to tell you what "did not work". I'm going to tell you what did work. And this was a hint from some guy in an opensuse forum.
The main point of this is that you have to deactivate the nouveau-drivers to make the nvidia-drivers be activated/working. The first advise was this:
Deactivate the nouveau drivers by setting the variable "nomodeset" in the grub installer - in other words:
When booting, press `e' to edit the bootloader...
- Find the line starting with `Linux'.
- Press `Ctrl-E' to get to the end of the line.
- Enter ` '(space) and write `nomodeset' (See picture below: 'nomodeset'.
- Press the key "F10" to activate and boot(without the nouveau-drivers).
What happens when you `boot* is that the nouveau-drivers are not loaded and you can install the nvidia drivers. But of course: This does not solve the problem. This is just temporary. What you have to do is make this change "constant". And the only way to do this is to edit the config-file.
The advice from the article was to edit the /etc/default/grub.config...but for my case ...that was not the case. I was using GRUB2...I'm guessing that that was the problem...I had to edit the same line in "/boot/grub2/grub.cfg". So after installing the nvidia-drivers edit this line.
After that everything was ok :)
My reason for wanting the Nvidia-drivers installed is because of building and installing the xmrig-nvidia miner. To do this you have to get the Nvidia-drivers and the CUDA-drivers working. See this Building XMRig-Nvidia v2.8.3 on OpenSUSE Leap 15.0. (There's a newer version XMRig-Nvidia out so you should update to the newest source.)