I'm on Arch Linux, and after i upgraded to Qt 5.8 / Plasma 5.9, i get a black screen and only a mouse cursor, instead of a desktop.
I already tried the solutions from this thread: viewtopic.php?f=289&t=125034, but so far only downgrading to Qt 5.7 / Plasma 5.8.5 restores my desktop.
Any help appreciated.

Try to login with different user, or save settings somewhere and start fresh session to see does Plasma starts.

Look at arch wiki for adding new user, and if everything works, delete plasma settings from first user and start over.

Do you have execute permissions on your ~/.cache folder?

Are you trying to login to a Wayland session?

That's broken with Qt 5.8.0...
https://mail.kde.org/pipermail/distribu ... 00161.html

A normal X11 session should work fine though.

I have execute permissions on ~/.cache folder. And i use the standard X11 session. I will try to login a new user and see what happens...

I managed to fix the problem by setting 666 as file and folder permissions on the ~/.cache folder. However this excludes execcute permissions on folders, so next i tried 755 on folders and 644 on files and got the black screen again. It seems that some process lacks write permissions with 644. Now i get this weird behaviour that when i set execute permissions on some folders in .cache ( kwin, krunner, kmserver-logout-greeter) i get the black screen again.
It should be easy to reproduce it by changing folder permissions and then trying to use the shutdown/logout widget, if it crashes then you triggered it.