Does your system come up to the first screen (where you key in the cheats) and can you key them in then? If your keyboard is working at this point, the problem is probably showing up as soon as KDE (the graphica interface) starts. Once the KDE is up and running, try unplugging the USB connector for your keyboard, waiting about 15 seconds and then plugging it back in.

When I went from 3.9 to 5.0.1 I had the exact same problem. Never got it figured out in 5.0.1 and it sounds like 5.1.1 is doing the same thing to you.
FYI, my usb mouse worked fine, even when the keyboard didn't.