Also forgot to check the Camera Data node, it is also busted (at least partially, the view distance socket appears to work fine).

After some fiddling with your file I've found that if any part of a shader tree has the bottom socket evaluate to an "invalid" material (IE from an empty shader input socket) AO will break on that material.
This does not occur if the top input is "invalid", AO will be fine regardless of input or lack of input.
This is true for both mix and add.