What units of measurement are you using? If you are using metres, the AO bias works out at about one millimetre. If you are using feet or inches it's even less. 

Is there any bump or displacement on the walls? With high surface unevenness, low AO bias, and that many AO samples, you may well be getting self-shadowing across most of the wall surface, and the lighter bit is where (for some reason) this self-shadowing is not happening.

I would try reducing any bump/displacement, and perhaps increase the bias to, say, 0.1 inches (converted to whatever units you are using) and do a spot render to see what happens.

But wait for others' responses too, as I may be way off the mark.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)