Chroma key was working perfectly yesterday, it's going berserk today.
PTZ Optics cam, green paper background evenly lit
Select the camera as input, then add the chroma key by selecting the paper color, just as we've done before when it's worked fine
But instead of blacking out the key color, it gives a very pixelated fuzzy key over the entire picture. Have tinkered with every possible setting we can see in the chroma key dialogue, and it does not fix it.
Have tried keying one still image over another one (i.e. to key something other than our green screen) as a test, and it does the same thing when we select a color in an image to key out. Will not let us key one image over another correctly.

I cannot possibly figure out what happened!

It looks like the Luma Key slider may have been turned up by mistake.
Take it back down to 0, then try the [1],[2] or [3] chroma key presets and everything should come right again.