The source of this text is the (former) forum of GPS-Expert.nl
If you cleaned too much your GPS when deleting files, this is the warning you could get in the JaVaWa Device Manager (JDM):
The basemap is for most devices important, especially when zooming out far (large map scale), the basemap takes over and the map on the screen will draw much faster. And leave the map always on in your device!
In some models a missing basemap can also cause strange behavior with routes.
If you don't have this file anymore in a backup, you have download it from here.
So, get the right basemap and follow the below procedure to install the map in the correct location of your GPS device: