file upload: The given path"s format is not supported.

I get this error when trying to run a file upload function on my client's
server.

It works fine on my local server.

I can also create a directory via my code, but not upload a file.

I'm using asp.net 1.1 and forms authentication.

The client server's folder is set up for the user ASPNET to have write
access.

Anything else I should look at? Any idea what has to happen to get this to
work?

-Darrel