Screensaver lost functions since High Sierra upgrade, can't wake screen with keyboard
Try turning ON the following setting:


  System Preferences > Mission Control > Displays have separate Spaces: ON (checked)


Then log out and log back in again for it to take effect.

Not really a solution—more of a workaround—since maybe you want that setting disabled.  But I guess it might be a bug in High Sierra, because I don't think the Mission Control settings should affect the screensaver...