This error is thrown by SF when I attempt to select a business object from the property sheet of an SF Textbox on a web page that inherits from "ApplicationBasePage". Since it is looking for the code in my project, if someone could tell me how the "MicroFour StrataFrame Extensibility" is searching maybe I could help track down the problem.

Any help will be appreciated.  Thanks Terry C

Exception
  The ApplicationBasePage could not be located.  Please rebuild project.

Source     : MicroFour StrataFrame Extensibility

Stack Trace:
   at MicroFour.StrataFrame.Extensibility.BusinessObjectNameTypeEditor.EditValue(ITypeDescriptorContext context, IServiceProvider provider, Object value)

I have the same problem , but sometimes it occurs, sometimes it doesn't.

I don't know what is causing it. I followed the instructions given here, but it doesn't solves it. Sometimes I close and reopen VS2008 and it gets solved, but other times it doesn't

I am using Windows Vista Ultimate x64, VS2008 and SQL2005

This sounds very much like a standard AppDomain / multiple assemblies on the same machine issue.  Be sure that you do not find any other Extensibility and AddIn assemblies on your machine.