Brightness can't be changed for Laptop-Screen

[MaMicha]

Hello,
since the update from 13.07.2021 (I guess), the brightness for my laptop screen cant’be changed:

• Using FN-F5 / F6 the onscreen slider (ref. screenshot)
  
  
  
  shows up and I can the values between 0% and 13%; higher values are not possible, the brightness does not change.
• Using SystemSettings > Hardware > Energy > EnergySavings, I can change the values for “powered”, “onBattery” and “onBattery, while low” between 0 and 100% but the brightness does not change…

I found something similar here and there, but I don’t see the solution or my knowledge is to poor, to follow.

$ Inxi

$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
   1920x1080     60.00*+
   1680x1050     60.00  
   1280x1024     60.00  
   1440x900      60.00  
   1280x800      60.00  
   1280x720      60.00  
   1024x768      60.00  
   800x600       60.00  
   640x480       60.00  
HDMI-A-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)

[MaMicha]

In addition to the above I found, that also the LED in the F4-key, indicating the microphone-muting, does not work. Pressing FN-F4 mutes / unmutes the microphone, the LED is always OFF.
I tried running other Operating Systems, even an older MANJARO-version: Both issues (screen brightness and F4-LED) only occur with the actual Manjaro.

Definetely, I need help with this!
Thank you.

[MaMicha]

My apologies for the noise, it has been my fault: In order to prevent from the message

$ systemctl --failed
systemd-backlight@backlight:acpi_video0.service

I added for a LENOVO T14s (AMD, 1st. Gen) based on Arch-Wiki that parameter “acpi_backlight=vendor” into /etc/default/grub:

GRUB_CMDLINE_LINUX_DEFAULT="text apparmor=1 security=apparmor ... udev.log_priority=3 noplymouth **acpi_backlight=vendor**"

This causes the issue; I removed it, brightness changes are possible now!