First of all I got it to work.
What happened was I was trying so hard to find the setting that was wrong that I forgot to check if it worked.
But to your question about XMP, this was the native DRAM Voltage after clearing the CMOS which I changed after changing the Core Ratio.
I found the Post you mentioned too but these steps didn't help.
I also installed the latest BIOS which helped neither.
After all I found the wrong setting and changed it. But don't ask me what it was since I change many things at the same time.