I was getting this error on a brand new site. The solution was to copy all the DLLs from the bin folder under: C:\Program Files (x86)\Telerik\Sitefinity 4.1\_EmptyProject\bin to my bin folder.
There's this behaviour that I'm seeing once you load up the project in VS 2010 and recompile, it actually NUKES dlls that are necessary to run the project!
The DLLs that kept getting deleted were:
I put them back, added the whole bin folder to source control, so at least when I compile the files won't be deleted.
------ Rebuild All started: Project: SitefinityWebApp, Configuration: Debug Any CPU ------
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(3607,9): warning MSB3061: Unable to delete file "C:\Projects\WebSite\bin\Facebook.dll". Access to the path 'C:\Projects\WebSite\bin\Facebook.dll' is denied.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(3607,9): warning MSB3061: Unable to delete file "C:\Projects\WebSite\bin\Newtonsoft.Json.dll". Access to the path 'C:\Projects\WebSite\bin\Newtonsoft.Json.dll' is denied.
SitefinityWebApp -> C:\Projects\WebSite\bin\SitefinityWebApp.dll
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========