The ASP.NET worker process is trying to read, write and delete copies of the safileup.dll in the ASP.NET temporary files directory:
C:\[%WinDir%]\Microsoft.NET\Framework\[version of the Framework]\Temporary ASP.NET
Give the account under which the application is running "modify" NTFS permissions on the ASP.NET temporary files directory. Be certain to check "reset all child permissions". Do this by clicking the "Advanced" button on the "Security" tab in the file "Properties" dialog.
To be certain of the identity under which your application is running, you can use the System.Security.Principal class to output the current user identity. For a code sample demonstrating how this is done, please see: http://support.softartisans.com/kbview.aspx?ID=820