sage notebook will not start after switching to Mavericks / Yosemite

edit

After purchasing a new Mac and transferring my data, the sage notebook fails to start on the new machine. I can run sage in the terminal however. The new Mac was originally running OS X Mavericks. I have now upgraded to OS X Yosemite. The notebook will still not start.

Does anyone know what's wrong? I would really appreciate a reasonable hint where to start looking for the cause of the problem.

The log output is copied below.

┌────────────────────────────────────────────────────────────────────┐
│ Sage Version 6.3, Release Date: 2014-08-10                         │
│ Type "notebook()" for the browser-based notebook interface.        │
│ Type "help()" for help.                                            │
└────────────────────────────────────────────────────────────────────┘

Please wait while the Sage Notebook server starts...
Traceback (most recent call last):
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/src/bin/sage-notebook", line 34, in <module>
    notebook()
  File "lazy_import.pyx", line 358, in sage.misc.lazy_import.LazyImport.__call__ (build/cythonized/sage/misc/lazy_import.c:3000)
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/notebook/notebook_object.py", line 228, in __call__
    return self.notebook(*args, **kwds)
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/notebook/run_notebook.py", line 529, in notebook_run
    nb = notebook.load_notebook(directory)
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/notebook/notebook.py", line 1811, in load_notebook
    nb = Notebook(dir)
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/notebook/notebook.py", line 150, in __init__
    S.load_users(self._user_manager)
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/storage/filesystem_storage.py", line 267, in load_users
    for user in self._basic_to_users(self._load('users.pickle')).itervalues():
  File "/Applications/Sage-6.3.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sagenb-0.10.8.2-py2.7.egg/sagenb/storage/filesystem_storage.py", line 167, in _load
    result = cPickle.load(f)
EOFError