Racked my brain for hours for the simplest of fixes....
I tried all of the following:
1) Reinstalled windows (even upgraded to 11)
2) Reinstalled League (in the drive it was in orginally)
3) Updated BIOS
4) Uninstalled Nvidia App
5) Switched off 3rd part app for my mouse, headphones etc.
Nothing worked
After all of that, I redownloaded League to my C drive after making some space. During that a .NET framework update appeared for the first time when downloading league.
Moral of the story, fix was definitely .NET framework, check windows optional updates if you do not see it, that should work as well.