Just spent 10 hours trying to figure out why a WorldPay callback to my development machine was not working.
This was simply because I had “Email scanning” protection enabled in AVG Antivirus. If you turn this OFF, everything will work fine.
This is related to the Stack Overflow post: http://stackoverflow.com/questions/7439063/iis-asp-net-website-managedpipelinehandler-error-an-operation-was-attempted