OK – so this time I deactivated the older version of Stream, and then deleted the plugin, and then reinstalled 3.0.0 from the WordPress repository. Failed again, same result. Just above the Plugins list in the dashboard, I see the red-bar notification message:
class wasn’t loaded because it doesn’t implement the get_label method.
I had to deactivate and delete the plugin to restore my site operation.
Seems like this one may not be ready for 4.3?