The DateTime Value is lost when using a custom format "dd.MM.yyyy" in RadgridView and custom provider is used. 

Workaround: 
Set the MaxDate in the custom provider:
Public Class MyMaskDateTimeProvider
  Inherits MaskDateTimeProvider

  Public Sub New(mask As String, culture As CultureInfo, owner As RadMaskedEditBoxElement)
        MyBase.New(mask, culture, owner)
        Me.MaxDate = DateTime.MaxValue
    End Sub

   
End Class