Yep, it was exactly this!!!
Had it as well, I just waited and it booted after a while. Never had it afterwards, only on the reboot after install.
But I donât have HDR on my monitor⌠would this fix work for me anyways?
It didnât woooork
I waited for around 8 minutes and nothing was happeningâŚ
I made the mistake of ticking the HDR box on our loungeroom/gaming system (EOS) and I get the blackscreen + cursor of woe now 
The offered fixes of editing (and making immutable) or renaming kwinoutputconfig.json, sadly, have made no difference. Itâs X11 for nowâŚ
Found this about spectacle on Fedora:
https://www.reddit.com/r/Fedora/comments/1drz4d6/spectacle_died/
:edit:
Do not install libva-vdpau-driver - that absolutely screws spectacle and took me an hour to find where I fscked up.
Ignore me, I forgot to stick them in IgonorePkg.
Just as a reminder: my issue is not SDDM or Plasma related. The black screen happens for me while the booting is still in text.
I have libva-nvidia-driver installed.
https://bugs.kde.org/show_bug.cgi?id=481977
Comment 16.
Watching footy 
It was libva-vdpau-driver that caused spectacle to screw up not libva-nvidia-driver.
I installed it - trying to get 555 drivers to run spectacle.
I had similar issue. Black screen and single monitor etc after 555 update. The solution for me was 6.6 LTS kernel. Just change your kernel to 6.6 LTS and run sudo reinstall-kernels command.
I did that already and it did not work. Maybe because I already tried it with 555 installed before hand. I think I should try installing the LTS kernel first before upgrading from 550 to 555.
Another trick is to use non nvidia driver again then switch to nvidia after restart. It will surely work. Just make sure you use LTS kernel. The thing is, it will work automatically. We just have to switch drivers.
Update: I tried the following steps:
- Uninstalling Nvidia 550 related packages and rebooting
- Installing the LTS kernel and rebooting
- Installing nvidia-dkms 555 and rebooting
Unfortunately, I still ended up on a black screen. I guess the LTS kernel isnât the answerâŚ
Have you tried backup and remove the config file?
mv $HOME/.config/kwinoutputconfig.json $HOME/.config/kwinoutputconfig.json.bak
You may need to reboot after backing up removing the file.
Well, I remember removing the file but not moving it to a backup. I should try this in case it does anything different. Even if I donât have HDR.
Actually wait, are you certain that this will work even if the black screen happens before the graphical interface is initialised?
Do you have nvidia_drm.fbdev=1 in the command line?