The fact that it’s an ASP.net app is probably exactly what the issue is here. Your application pool identity has to have rights (not necessarily the IIS identity; by default, the app pool identity is the local Network Service account.) You also probably need to run caspol.exe on your IIS machine.
http://msdn.microsoft.com/en-us/library/cb6t8dtz%28v=vs.80%29.aspx
http://learn.iis.net/page.aspx/50/aspnet-20-35-shared-hosting-configuration/
%windir%\Microsoft.NET\Framework\v2.0.50727\caspol -m -ag 1. -url "file://\\remotefileserver\content$\*" FullTrust