I am pasting once again the reply I already provided in the support ticket:
With this property set to false the grid will not use LINQ queries behind the scenes to execute built-in operations like sorting, paging, filtering, etc.
Unfortunately we cannot commit to a timeframe for a fix because the LINQ itself appears to support operations with null values for dates in a bit different way -see the resources below for details:
Once this logic changes, the grid will automatically support them as well.
the Telerik team