OpenStreetMaps recently changed their server infrastructure to better handle the load. This had the side-affect of breaking all OSM-based maps in MyTrails because they now require a specific per-app User-Agent HTTP header.
Unfortunately MyTrails didn’t support directly editing the HTTP header so fixing the maps required updating the app.
The maps should be updated when you launch MyTrails after the update, unless you already changed an affected map, in which case MyTrails will create a fixed copy at the end of the list.
A change on the Minambiente site broke this map for MyTrails. Fortunately, a simple edit fixes it: MyTrails > Maps > tap on the IGM 25k (IT) line and in the map details editor, change the version from 1.1.1 to 1.3.0, then use the back button to save the change.
This will be available by default in the next beta release of MyTrails.
Update June 2019: the former IGM site disappeared entirely and I can’t find another source of high quality topo maps. I was able to find a server that offers black and white maps, but they’re broken down by region, so I can’t include them as-is in MyTrails. Please join the forum to join the effort to replace this map!