Due to new render window names.
The issue was already mentioned in T26545: [Multi widget] Make classes work with QmitkAbstractMultiWidget(Editor) and T29203: [Multi widget] Make classes work with the new render window names but will be tackled separately here after the next release.
The IGT classes (e.g. QmitkUSNavigationMarkerPlacement) need to be refactored and tested. Since this is a broader topic, this issue was decoupled from the mentioned task.