After the stable update of 16 December 2021
[Stable Update] 2021-12-16 - Kernels, KDE Gear 21.12.0, Gnome Extensions, LibreOffice
LightDM is working again, but I’d like to wait to set this as solved.
EDIT 29 December 2021:
After several reboots all is fine: LightDM still works without issues.