sddm: open new session gives black screen

Project:Linux software
Category:bug report
Related pages:#9199: Simple Desktop Display Manager (SDDM)
Tags:Black screen, Calculate Linux, sddm
I am a new Calculate Linux user.
I have just successfully installed Calculate.
I can log into a KDE Plasma session, with sddm and the nvidia-drivers.
However, when I try to start a new session, I get a black screen. I cannot switch to any tty (tty1 nor back to tty7). I can only hard-reboot the computer.



It's not a user permission problem because the bug manifests itself before the user has a chance to log in.
I can reproduce the bug in two ways:

1] I log in and start a KDE session normally on tty7. I clik K-menu > start new session and I get a black screen from which I cannot get out. sddm never gets to display a login screen.

2] After booting, before I log in any session. I see the normal sddm login screen on tty7. I do ctrl+alt+f1 and see a CLI prompt to login. I do ctrl+alt+f7 to get back to the graphigal login, but I get the black screen instead and cannot return to either tty1 nor tty7. Note that I never logged in anywhere.

Which component may be broken? agetty?