Could not find a part of the path

I know this has been discussed a lots but I just can't fix it so far:
My web app is running under defaultapplicationpool and the identity is set to local system, I need to write some text into a folder and I added "Authenticated User" to the folder with write permission.
Why I am still getting error: Could not find a part of the path?

Dim LogPath As String = System.Configuration.ConfigurationManager.AppSettings.Item("LogPath") Dim file As System.IO.StreamWriter Try file = My.Computer.FileSystem.OpenTextFileWriter(LogPath + DateTime.Now.ToString("yyyy-MM-dd") + ".log", True) file.WriteLine(sDescription) file.Close() Catch Ex As Exception Throw Ex End Try
The environment: Windows 7, IIS 7.5
Thank you very much.