Please, can someone tell what this is about?
This problem has existed an unusual long time without being fixed. Some people have problem with kde and plasma, some as I understand has problem with older nvidia graphic cards.
As all who have problem can fix i temporarily with an older xserver is it the same thing that need to be fixed, ie xserver or is more things like nouveau drivers?
If it not will be fixed for my computer with older nvidia card maybe I shall install something else on that because this is the family's main media center for music and tv in our house and I want no problem with that.
Had the same problem as you, with failing login in xfce.
It seems that xserver and xfwm4 compositing are not playing nice.
While on downgraded xserver 1.19, I disabled compositing:
xfconf-query -c xfwm4 -p /general/use_compositing -s false
then updated xserver to 1.20 - and rebooted.
Suggest you then install compton, which may be a better compositor anyway to cure tearing on older video cards. It will at least offer you compositing until a xserver or xfwm4 are patched