The LoginForm widget used to work on a different way (export of i18n by the xas), because the structure changed so did the widget. A new version will be available in the App Store today or tommorow.
New LoginForm Widget is available in the App Store. This widget is not compatible with Mendix version before 2.5.1.
Did you use the same theme in two different projects? Like one in 2.5.1 and 2.5.0? Did you emptied your cache?