These guides: HowTo Find error logs and Checking for errors by reading logs or shell output should be useful.
As already suggested, optimus-manager-plasma may be worth trying instead of optimus-manager, considering you’re apparently using KDE Plasma. Try that and see if it makes a difference – if not, then the issue is something else; likely graphics-related. You can also try removing any plasmoids/widgets/extensions you have loaded. Cheers.