I just installed the plugin in a new website, not yest visible to the world but visible by a spoofsetting in the host file.
I get this message. But the icons are in the folder and the folder exists.
i did not move the website, nor did resetting to defaults also does not resolve the issue.
Leaflet Maps Marker Warning: the setting for the marker shadow url (http://mywebsite.com/wp-content/uploads/leaflet-maps-marker-icons) seems to be invalid. This can happen when you moved your WordPress installation from one server to another one.
Please navigate to Settings / Map Defaults / "Default values for marker icons" and update the option "Shadow URL". If you do not know which values to enter, please reset all plugins options to their defaults