debian lxqt sddm greeting fail after nvidia driver install

versions:

debian: 9.5 (stretch)
kernel: 4.9.0-7-amd64
sddm: 0.14.0-4+deb9u1
xserver-xorg: xserver-xorg-core 2:1.19.2-1+deb9u5
desktop: lxqt 0.11.1-1

==problem:== working system failed to start the greeter to login to the desktop environment after install the proprietary nvidia drivers. the screen is simply black. if stopped and started manually with "startx", the desktop environment works.

solution:

1. dkms autoinstall

and reboot...