Could be read/write errors across the network. Audiobook Builder's a pretty intense writer (and no slouch at reading) during a build - and a few extra concerns come up when building across a network. Much of the writing is under QuickTime's control...and when it fails we don't usually get to know why. Lets see what we can dig up. Can you get Audiobook Builder to fail again, then send a full System Profiler report to
support@splasm.com? To create the report launch /Applications/Utilities/System Profiler.app, choose "Full Profile" from the View menu, then select "Save..." from the File menu.