CRM 2011 Update Rollup - Error after installation

After installing a CRM 2011 Rollup update, you may in some rare cases get the following error when trying to launch the CRM web application:

An error has occured.

Try this action again. If this problem continues, check the Microsoft Dynamics CRM Community for solutions or contact your organization's Microsoft Dynamics CRM Administrator. Finally, you can contact Microsoft Support.

Solution
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.Xrm.Sdk.dll

Microsoft.Crm.Extensibility.dll

Using 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

3. iisreset

That's it, hopefully your CRM should be back to its normal self!