Overload Error

I have developed an error in a web application that uses the progstudios.webcontrols.combobox (asp.net 1.1) The strange thin is that I have at least 20 back up copies of the same application taken over the years and they all now give the same error, so I can rule out an accidental code change.
The error is -

Overload resolution failed because no accessible 'LoadListControl' can be called with these arguments:
    'Public Shared Overloads Sub LoadListControl(ddl As ProgStudios.WebControls.ComboBox, ddlSource As Object)': Value of type 'ProgStudios.WebControls.ComboBox' cannot be converted to 'ProgStudios.WebControls.ComboBox'.
    'Public Shared Overloads Sub LoadListControl(ddl As System.Web.UI.WebControls.ListControl, ddlSource As Object)': Value of type 'ProgStudios.WebControls.ComboBox' cannot be converted to 'System.Web.UI.WebControls.ListControl'.


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