I was not able to solve this problem until I finally granted access to "Everyone" on the .../Temporary ASP.NET Files folder.  However, when I looked into this, I found this worked because my application was impersonating a specific service account.  I would encourage everyone to clearly identify the user that the process is running as!  Does your web app impersonate the current user?  If so, it would likely work on your development machine, where you are an admin, but you would see this error on a server where you didn't have privileges to the temporary asp.net folder.  Also consider what identity your app pool is running under.