If the code of the Silverlight.Controls is defined in another custom assembly you need to add an assembly token (see
Mapping to custom assemblies[
^]). For instance if the namespaces are defined in the AbcXxxYyySilverlight.dll:
xmlns:local="clr-namespace:ABC.XXX.YYY.Silverlight.Controls;assembly=AbcXxxYyySilverlight"
xmlns:login="clr-namespace:ABC.XXX.YYY.Silverlight.LoginUI;assembly=AbcXxxYyySilverlight"