I assume that you are uploading the file to a network share folder '\\platon2\greffe2\09-123\'.
If you are running your ASP.NET application, then you need to have read/write access for the 'YourMachineName\ASPNET' account to the shared folder.
I also assume that you might given access only at folder level and not for sharing permission.
If my assumtion is correct, set set access rights to the folder sharing also.