Ipython: IPython Notebook-Fehler: Fehler beim Laden des Notebooks

In den letzten Minuten konnte ich auf keine der zuvor mit Python3.4 erstellten .ipynb Dateien zugreifen. Ich gehe in das Unterverzeichnis mit diesen Dateien, gib ein

ipython3 notebook

Wenn ich auf eine bestimmte .ipynb Datei klicke, erhalte ich die folgende Fehlermeldung:

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

Wie löst man dieses Problem?