Ipython: Erro do notebook IPython: erro ao carregar o notebook

Nos últimos minutos, não consegui acessar nenhum dos .ipynb arquivos criados anteriormente com Python3.4. Eu vou para o subdiretório com esses arquivos, entro

ipython3 notebook

Quando clico em um determinado arquivo .ipynb , recebo o seguinte erro:

An unknown error occurred while loading this notebook. This version can load notebook formats v4 or earlier. See the server log for details.

[E 00:19:41.468 NotebookApp] Unhandled error in API request
    Traceback (most recent call last):
      File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/traitlets/traitlets.py", line 460, in get
        value = obj._trait_values[self.name]
    KeyError: 'db'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/notebook/base/handlers.py", line 458, in wrapper
    result = yield gen.maybe_future(method(self, *args, **kwargs))
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/tornado/gen.py", line 1008, in run
    value = future.result()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/tornado/concurrent.py", line 232, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 3, in raise_exc_info
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/tornado/gen.py", line 282, in wrapper
    yielded = next(result)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/notebook/services/contents/handlers.py", line 124, in get
    path=path, type=type, format=format, content=content,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/notebook/services/contents/filemanager.py", line 354, in get
    model = self._notebook_model(path, content=content)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/notebook/services/contents/filemanager.py", line 314, in _notebook_model
    self.mark_trusted_cells(nb, path)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/notebook/services/contents/manager.py", line 447, in mark_trusted_cells
    trusted = self.notary.check_signature(nb)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/nbformat/sign.py", line 220, in check_signature
    if self.db is None:
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/traitlets/traitlets.py", line 488, in __get__
    return self.get(obj, cls)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/traitlets/traitlets.py", line 467, in get
    value = self._validate(obj, dynamic_default())
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/nbformat/sign.py", line 127, in _db_default
    self.init_db(db)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/nbformat/sign.py", line 139, in init_db
    )""")
sqlite3.DatabaseError: database disk image is malformed

Como resolver esse problema?