Upgraded site from 3.5 to 4.0 - Getting an error

Question

I figure there's something in the web.config I need to switch?

[A]System.Web.Configuration.ScriptingAuthenticationServiceSection cannot be cast to [B]System.Web.Configuration.ScriptingAuthenticationServiceSection. Type A originates from 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Extensions.dll'. Type B originates from 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll'.

Answer 1

You probably have a stray reference to System.Web.Extensions v3.5 somewhere in Web.config.
(most likely in <configSections>)

Remove or replace it.