@thomass : Small workaround until it is fixed: Close the view and open it again. The image and segmentation are autoselected and the tools are active.

Don't ask me what the reason is, that the problem appeared now! But the enabled state was only set at view construction for the ML view. Which lead to disabled tools if you opened it the first time (afterwards a reference image was set in the toolmanager and everything was fine).
Normal segmentation worked arround it by recreating the buttons as soon as data was changed. But I think it is cleaner to let the toolbox widget update the states whenever the toolmanager data has changed.