Only solution I know of when an error like that happens is stop the web site in IIS then delete the folder:
c:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\fcdc6d38\90757c7d\
then start the site again
Something got funky with the temporary files, by clearing them out they will be re-created, but you won't be able to delete that folder without stopping the IIS web site.
Hope that helps,