I'm running borderless mode. Fullscreen definitely kills my program, I'm fully aware of that.
My headset is broken - or rather, the mic is broken. So I'm using a pretty old, but still good, standalone mic, but it really is nothing but a mic, lol. So muting it has to be done through whichever voice communication program I'm using.
It doesn't matter much anyway. My program is working fine.
I had just forgotten that I had to run LoL as administrator, because running it normally made the game crash instantly. However, when running it as admin, it's able to block far more programs, including mine.
I simply had to re-run it normally and everything got back to normal.