4UMaps are some of the nicer OpenStreetMap-based maps, maintained and served independently of OSM (and MyTrails).
They have recently updated their server, and some versions of Android seem to have trouble connecting to the new server. You can fix this by changing the map definition for 4Umaps to use the following URL:
Alternatively, just tap this link from your Android device to import a new map definition with this URL.
Update: the 4Umaps URL changed again. And again…