Brightness adjustment Fn keys stopped working in 18.04
Solution 1:
On Xubuntu 18.04 (Ubuntu with xfce desktop) my brightness problem finally solved.
I opened 'Onboard Settings' under the 'Settings' menu. If your desktop doesn't have one, you can download it from Software Center.
On the 'Onboard Preferences' window, click the Layout menu, then choose the 'Full Keyboard' (Desktop keyboard with edit and function keys). Click here to view the Onboard Preferences window