Bug#797979: Again
I have discovered it is somehow related to the dual monitor settings. When I plugged external monitor before login (during a time when the sddm is on screen) I was able to start plasma desktop in dual monitor mode. But then when I unplugged the monitor, then notebook LCD gone black and only thing you can do is reboot (plugging in external monitor doesn't help). So I repeated the previous step with the external monitor plugged in during login. (Frankly I didn't try login without but I tested that unplugging ends with black screen again.) But then I rebooted with the external monitor plugged in but when grub appeared I unplugged the external monitor and was able to login and it works now!
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