DayPilotMonth is undefined orDayPilotCalendar is undefined.
SImple solution, my website was set to ASP.NET 1.1 however it needed to be 2.0.
Change the setting in IIS on the default website and run a 'iisreset'
Go to the properties of the actual website that is hosting the application.
Go to home directory / configuration / axd extension - open, then OK.
Choose OK to cascase.