It is though, at line 297.
You're right, I've look over that.
I work with OP on the same project, and the thing that still baffles us most is that the code runs on the older Arduino/Ethernet Shield build but not on those two other builds, especially considering that those are newer (but have the same name, just different packacking).
Do you still have the packaging? Is it an original Arduino or is it a clone (are there already clones of the Due)? Is the Ethernet Shield the same or also a different version? If it's different, what revisions do the two versions have? Can you make pictures of the hardware and post them?