Did you verify you installed Leaflet/OSM correctly? Maybe check it again
Any chance you tried changing the map or map provider?
Did you change local.config - Config::Set ('MAP_TYPE'... to reflect "OpenStreetMap.Mapnik"? Line should read:
Config::Set('MAP_TYPE', 'OpenStreetMap.Mapnik'); <---- need this one
And the old line either deleted or commented out.... //Config::Set('MAP_TYPE', 'G_HYBRID_MAP');