ASP.NET ConfigurationErrorsException when serving images via UNC on IIS6

I have a virtual directory in my web app which connects to a Samba share via UNC. I can browse the files via Windows Explorer without issue, but my web app throws a yellow screen with the following message:

Description: An error occurred during the processing of a configuration file required to service开发者_如何学编程 this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: An error occurred loading a configuration file: Could not find file '\cluster\cms\qa-images\120400\web.config'.

What makes no sense to me is why it's looking for a web.config in that location. I know it's not an authentication issue because the virtual directory can serve images from its root (i.e. \\cluster\cms\qa-images\test.jpg serves as http://myserver/upload/test.jpg just fine).

Ideas?