Each worksheet process now loads $DOT_SAGE/init.sage (see #8092 for a bug fix we merged between Sage 4.3 and 4.5.2). If you call notebook() in this file, then each worksheet will try to start a new server in the default server directory. (You can change this with the directory keyword option, although that's likely not what you want here.) To start a notebook server, I usually run a bash script that contains, e.g., sage -notebook interface='' open_viewer=False port=XXXXX secure=True  By the way, these and other options are described in notebook?. Each worksheet process now loads $DOT_SAGE/init.sage (see #8092 for a bug fix we merged between Sage 4.3 and 4.5.2). If you call notebook() in this file, then each worksheet will try to start a new server in the default server directory. (You can change this with the directory keyword option, although that's likely not what you want to do here.)
sage -notebook interface='' open_viewer=False port=XXXXX secure=True

By the way, these and other options are described in notebook's docstring, accessible via notebook?.