There is a new issue on newer phones (Lumia 550, 640, 950) which got the latest Windows Mobile System update. The app crashes if you open the map. This only happens if you have selected the HERE map for the map, which is the default. The reason is not yet known.
You can use the Online maps instead. Select Main screen / Download / Online / Normal and use this selection for the map with the OK sign. The last selected map provider will be saved for the next start of the app.
For offline usage you can use the offline vector maps. Read the instructions in the HowTos how to install them.
Sorry for any inconvenience.