SFTP starts returning -1401 errors after long connection period in 1.3.4

I have an SFTP connection to a web host, and sometimes after long periods of inactivity, it will start giving an error in the Finder when I try and open a file. It says:

The operation could not be completed.
An unexpected error occurred (error code -1401).

Attempting to save a file to the drive gives errors in whatever program I use. Also, the icon on the drive in the sidebar changes from the red SFTP/Expandrive icon to a generic Folder icon, but the icon on the desktop stays as it should be.

If I cd to /Volumes/MySharedDrive in Terminal, then do an ls, I get:

ls: .: Device not configured

Interestingly, Expandrive hasn't figured out that something's gone seriously wrong, and leaves me with the ability to traverse the directory structure in the Finder - I just can't open or save any files. Annoyingly, Finder tells me that the drive is in use, and so I can't easily Eject it.