StepValue is defined as Int32, so 1000000 should not be a problem.
I first only noticed this problem in Visual Studios XAML designer, but at runtime (running at German locale) there were no problems. But changing the locale to "English" I now get a runtime exception which kills my app immediately, although we are handling nearly every unhandled exception with an error-report dialog:
System.Windows.Markup.XamlParseException occurred
  HResult=-2146233087
  Message='Set property 'ActiproSoftware.Windows.Controls.Editors.Parts.Primitives.Int64PartBase(System.Nullable(System.Int64)).StepValue' threw an exception.' Line number '58' and line position '8'.
  Source=PresentationFramework
  LineNumber=58
  LinePosition=8
  StackTrace:
       at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
       at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
       at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream)
       at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
       at DotNetFabrik.DoublePics.Cockpit.MainWindowParts.PartMainParts.PartSearchPreparationParts.FileHashSearchPreparation.InitializeComponent()
       at DotNetFabrik.DoublePics.Cockpit.MainWindowParts.PartMainParts.PartSearchPreparationParts.FileHashSearchPreparation..ctor()
  InnerException: System.ArgumentException
       HResult=-2147024809
       Message='1000000' is not a valid value for property 'StepValue'.
       Source=WindowsBase
       StackTrace:
            at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
            at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
            at MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(Object inst, XamlMember property, Object value)
       InnerException: