Failed to load resource: the server responded with a status of 404 (Not Found)
http://path/to/the/tile//base/8/6/53.png
etc.

var baseLayer = new ol.layer.Tile({
  source: new ol.source.XYZ({
    url: 'http://path/to/the/tiles/base/{z}/{x}/{-y}.png'
  })
});