If you have just completed an upgrade of your CRM system from 3.0 to 4.0, you might have discovered that your 3.0 Callouts are no longer working properly. This can be a very annoying problem, as most organizations do not want to upgrade all of there server side code right away, and need it to work immediately as promised. One of the simplest ways to debug this issue is to make sure that the Microsoft.Crm.Platform.Callout.Base.dll has been registered in the GAC during the upgrade process. If this wasn't successful, all of your 3.0 callouts will fail. Often with cryptic and unexplainable error messages. If you see that this dll is not registered in the GAC, simply register it and run an IISReset. Then retest your code. I would be willing to bet that this solves your problem.
The assumption to all this, of course, is that the callouts were working properly before the upgrade!
