I got it!
After having a delicious homemade pizza 
and a suitable amount of red wine 
, I tried some more tests. Things I didn’t dare to try before, due to the lack of documentation. I really hate tapping out in the dark without any information. I already tried any combination of soft, hard, warm, cold, bios, acpi and force. But not pci! 
My solution works like a charm. Adding the kernel param reboot did the trick. Append reboot=pci to the variable GRUB_CMDLINE_LINUX in /etc/default/grub.
GRUB_CMDLINE_LINUX="reboot=pci"
Finally, a huge thanks to all contributors. Absolutely great being part of such a community!