First, I see the same question posted in December 2008, but seemingly not answered.
Creating my first Master Page in C#, the ContentPlaceHolder gets the error message:
"Error Rendering Control - ContentPlaceHolder1
An unhandled exception has occured
This control can only be used in a MasterPage."
It seems that Visual Web Developer Express does not fully recognise the page as a Master Page. Anyone who can help in resolving this?