I managed to replace my Mainboard and CPU, but I still get the same error as before. After all the trouble with the Mainboard, I hoped the issue is now gone and it was the Mainboard. My PC is now less crashing than before, but it seems like my GPU is really damaged somehow.
I added the rest of the money to this goal. I hope that the GPU prices go down in the near future. I'm setting the goal to the current price.
Thanks to all the people that support me and help me out! I love you guys!