Still reproducible (XFC4, nvidia, switch user, black screen) after updating to:
[Stable Update] 2024-05-13 - Kernels, Plasma 6, Gnome 46, LxQT 2.0, NVIDIA
Checked logs again (as root, or logged in ‘black screen’ user):
journalctl -wtail -f /var/log/* /var/log/*/*tail -f ~/.xsession-errors
Nada de nada…
Troubleshooting hints still welcome…
I guess nobody is using the switch users feature 
Workaround is to:
- logout from the first user before logging in as the second one
- if happens use ctrl-alt-backspace to kill the session