You can resolve this very easily by performing the following steps:
1. iisreset - if this has not resolved the issue then try restarting the server
if the above steps have not fixed the problem, then In brief the issue is that certain assemblies have not successfully been updated in the GAC (Global Assembly Cache)
2. GACUTIL (.net framework 4) to install the following assemblies:
Microsoft.Crm.Extensibility.dllUsing the command prompt either from Visual Studio or from the Windows SDK tools, perform the following commands from within the CrmWeb\bin folder.
gacutil /i Microsoft.Xrm.Sdk.dll
gacutil /i Microsoft.Crm.Extensibility.dll