FWIW, this setting is set in the "wp_options" table of the database (option name=wpgmza_xml_location). I copied my live development site back down to my local computer and am receiving this error as well.
Setting your permissions on "wp-content/uploads/wp-google-maps" to 777 is fine and dandy but it will never solve your problem because the plugin doesn't appear to update this database settings if you move your site. I tried deactivating/activating the plugin but to no avail. I suppose I'll manually update the path in the database to solve my problem but that's probably not an ideal solution for a lot of people here.
To the plugin author, any thoughts?