You used to see screenshots of this more often. Some section of the map just didn't generate properly.

The difference here is that Blaine has already been through that section. He was on his way back to the red portal, and hit this like a brick wall. Or like a solid wall of... nothing! He couldn't continue, and also had some trouble opening a Town Portal. He tried it a few times, then finally heard the sound, but didn't see anything happen.

Finally, he ran back to the left, where a portal had opened up, some ten yards or so from where he had read the scroll.