I use a Shuttle PC with two old 24" Apple cinema displays. Since upgrading to Lubuntu 18 nearly every time the screensaver gets enabled for the first time during a day I am not able to wake the displays up. The PC is running fine, I occationally start an audio file with the keyboard or something. But the displays are not coming back. I have to press the power button and restart and then mostly the problem is gone. I already deactivated the energy saving mechanism so the screens only get "dark" and not shut down completely but there is no difference.