I've noticed the same thing in an interactive sessions (i.e. not under a web server).

I think it happens after the network mappings get disconnected after a period of time to conserve resources (but the mappings are not deleted). After that (or something else?) happens, accessing the drive letter from the console (e.g. dir w:) does not restore the connection and fails.

The connection can be re-established by accessing the drive letter through the GUI. After doing that, dir w: works again. I don't know of any programmatic alternative.