That message is a sign of the server closing the connection (surprise!). That's usually a sign of the server experiencing too much traffic or it specifically throttling/blocking you or your IP.
Mauling Snarks is over 300 chapters and over 1 million words. I follow it myself, but I haven't downloaded the whole thing at once since it had 11 chapters.
About the only thing I can suggest (assuming you're already doing epub updates and not downloading the whole thing every time) is to put a larger
slow_down_sleep_time setting in personal.ini. Default for xenforo forums is 4.
Code:
[forums.sufficientvelocity.com]
slow_down_sleep_time:6