Well the line that error occurs on is thus:
Code:
local offscreen = chatFrame.buttonFrame:GetTop() + BNToastFrame:GetHeight() + BN_TOAST_TOP_OFFSET + BN_TOAST_TOP_BUFFER > GetScreenHeight();
Any of those functions or values could be what they are talking about but being a blizz function it will be hard for us to test it out.
First port of call should be chatter as they are the calling addon. Whether nUI is changing a value chatter is using to tell blizz about battlenet stuff I don't know but hopefully they will be able to check the code back to see where the problem could be coming from.
So, basically it sounds like a compatibility issue between nUI and chatter in one of their last patches.