I have downloaded the latest ELMAH and copied the sample app’s Web.Config settings to my ASP.NET 3.5 Web.Config.
I have made sure to reference the 3.5 release version of the DLL, but I am still getting the above error (as per the title of this post).
If you have the reference then confirm you have an entry in your assemblies dictionary from the web.config that has the dll that contains Linq. For example: