Nvidia problem: Failed to assign any connected display devices to X screen 0

Hi there,

After having spent hours of trial and error I am still not able to solve this problem. I tried several solutions I found on the internet --> nothing worked for me. Now I really hope somebody here can help me.
I have a HPZ820 workstation which used to work well last week. Over the weekend I shut it down to save some energy.
When I tried to turn it on yesterday it wasn't able to boot to the graphical environment. There was only a blinking cursor at the upper left corner. Then I tried to boot into runlevel 3 which worked well. From there i tried to start X which failed. The error log is attached in the file Xorg.txt.
If I delete the xorg.conf I am able to start X. But everytime I write a new xorg.conf (which is done by nvidia with nvidia-xorg) X can't be started and the mentioned log file is written.
And that is my problem.
I have really no clue what is the reason for that since I didn't change anything besides a few updates from RH (The updates should be listed in the attached file screenshot.png). Before the weekend-shutdown everything worked well with the Nvidia driver. (It is the prop. Nvidia driver which is recommended for CUDA use.)

Do you have an idea what my problem could be?

Your help is really appreciated. Thanks a lot in advance.

Best regards, Stefan Arzbacher