Now I apologise to anyone who wants to learn some new computer skills but the solution turns out to be very "non-tech" - but I swear this worked for me as strange as it might sound.

Shutdown your PC. Now physically disconnect it from the mains power supply. This apparently clears any residual power on the motherboard. Leave it disconnected for a while (I recommend 30 minutes as that's how long I waited). Reconnect everything, boot up your PC and enjoy your fully functioning USB 2 ports and devices again!

Right now I'm able to use all of my USB 2 devices in the USB 2 ports without a problem. How long will this continue? No idea. But I know what to do if/when it happens again.

Notebook users with the same problem will need to remove the battery as well as mains power, but I understand this method will still work.