A valid SQL statement should look like

SELECT * FROM Products WHERE ([Discontinued] = 1)

or

SELECT * FROM Products WHERE ([Discontinued] = 'True')

but RadGrid provides something like

([Discontinued] = True)

in the MasterTableView.FilterExpression property.

By default RadGrid filters the data on its own after retrieving the entire data set form the data source, and the filtering operation works like that.
If you will be using the filter expression provided by the grid in a custom data source operation you may need to tweak the string first.
Note that changing this may result in a breaking change if you are already using the current syntax.

Workaround: https://www.telerik.com/support/kb/aspnet-ajax/grid/details/get-sql-compliant-filterexpressions-from-radgrid