We depend on an external web service to supply the map tiles. Apparently that server is having some difficulties. If you have another source of map tiles you can point your map to that source. Certainly, it's possible to build and deploy your own tile server, but we've not found it worth the trouble based on the fact that the tile server we use (
http://vmap0.tiles.osgeo.org/wms/vmap0) has been very reliable.
According to the mailing list they are having to do some server maintenance that took longer that expected. If it's not restored soon we can start looking into alternatives.