Map not visible

I have the device since a few days and was happy with it. However, since today the display of the map is buggy. Sometimes it works properly. Most of the times it shows no map at all, only the trailforks trails. If I change the zoom the map shows for the blink of a second, so I suggest it's on the device, but not displaying properly. Any suggestions what to do? I already did a factory reset, but the problem remains. If I have the time tomorrow I'll try to reinstall the latest software and map. Any other suggestions?