front end - Error when attempting to save Notebook. Cause and solution?

I am experiencing an error attempting to save a modified Notebook in Mathematica 10 under Windows.

The error I get is:

Could not access the directory X:\Docs\\Mathematica. You should check that directory's permissions.

Note the doubled \\. The correct path is X:\Docs\Mathematica.

NotebookDirectory[] returns the correct (escaped) path:

NotebookDirectory[]

"X:\\Docs\\Mathematica\\"

Correction: This is happening with multiple (all?) Notebooks in this path in the current session.

Has anyone else experienced this error? How can I determine its cause and seek a solution?