It seems to have been an intermittent error - it is working again.
When you see your live map, you see some cache icons and a map. When you select one of the options in tile layer settings, you see different maps. Those maps are the tile layers. When you pan or zoom the map, what you are viewing changes. What you see is loaded in as rectangular images known as tiles.
TLDR: Every option except OpenStreetMap from the tile layer settings menu resulted in no map being shown. It seems to have blown over by now.