Actually I was wrong. Max line width is a member variable, which is set via the lineWidth property. There used to be a "mLineWidth" field, but it has been deprecated a while ago, serving only backwards functionality.
I am not sure why commenting those two lines makes it work for you, considering that the fields are not supposed to be used anymore. Did you create your UI with an old version of NGUI (like free version?) -- You need to resave with the latest full version.