Brightness control no longer working after update to Ubuntu 23.10 in computer Dell OptiPlex 7780 AIO

I have updated to Ubuntu 23.10 from 23.04 (Kernel from 6.2 to 6.5) and brightness control in the computer Dell OptiPlex 7780 AIO is no longer working properly.

The kernel module dell_uart_backlight (which I think is responsible for the proper adjustment of brightness in this device) is loaded.

The brightness slider sometimes appears, and sometimes not. After locking the screen it always disappears, even if it was present before.

For example, the gnome command to increase brightness

gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp

returns:

GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gsd_5fpower_5fmanager_5ferror.Code1: No usable backlight could be found!

Maybe this is a bug in the Kernel? Thanks a lot for your help!

Jordi.