I had to turn CSM on and set SecureBoot to "OtherOS" to get the UEFI installer running (sometimes took 2-3 boot attempts).
Installation process was smooth.
At the beginning when I logged into Xen, my system froze and I could do nothing except to restart it. After I first thought that it may come from my NVIDIA card and I tested it by using the Onboard Intel Graphics chip, I later found out, that it was my old Creative Soundblaster PCI card which seemed to cause the crash. Once I removed it, everything was stable ever since.
Thank you for the Qubes-OS development!