The SolarSystemExplorer now is built on METoolkit v2.3 which is shipped with a new UI, several new features and bugfix. See the METoolkit Repo.
SolarSystemExplorer v2.3.0 only supports Unity 2017 if you are building it from source as METoolkit v2.3 is compatible with Unity 2017 only. The latest release that built with Unity 5.5 is SolarSystemExplorer v2.2.0.
This build requires MeshExpert Server v2.2 and above.
Note: You can download and run the compiled versions for PC, HoloLens, and Surface here. Also you can follow our documentation to build your own versions.