Los elementos SortDescription agregados no son válidos. Las soluciones posibles son establecer CanUserSort en la columna en false, usar la propiedad SortMemberPath en la columna o controlar el evento Sorting en DataGrid.
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English version
The SortDescriptions added are not valid. The probable solutions are to set the CanUserSort on the Column to false, or to use SortMemberPath property on the Column, or to handle the Sorting event on DataGrid.