Comment # 31 on bug 1151290 from Dr. Werner Fink

Adding the user sddm to the group video in /etc/group does solve the issue ...
it seems that sddm has to have the permissions to open the nvidia devices below
/dev

Also the line

  xrandr --setprovideroutputsource modesetting NVIDIA-0 && xrandr --auto

is not required anymore as the direct access of nvidia works in my ServerLayout
section

  xrandr --listproviders
  Providers: number : 2
  Provider 0: id: 0x243; cap: 0x1 (Source Output); crtcs: 0; outputs: 0;
associated providers: 1; name: NVIDIA-0
  Provider 1: id: 0x46; cap: 0xf (Source Output, Sink Output, Source Offload,
Sink  Offload); crtcs: 3; outputs: 4; associated providers: 1; name:
modesetting
    output eDP-1-1
    output DP-1-2
    output HDMI-1-1
    output HDMI-1-2

  glxinfo | grep 'vendor string:'
  server glx vendor string: NVIDIA Corporation
  client glx vendor string: NVIDIA Corporation
  OpenGL vendor string: NVIDIA Corporation