Seems there is a number of new people having issues with the Store Locator Plus® address lookup feature due to a failed REST API request. With Store Locator Plus 5 all address lookups are routed back through the WordPress site via the REST API in order to protect Google API keys.
If your site is running WordPress from a subdirectory you may run into issues if your web server is not configured to properly handle REST API routing. Especially if the site is using “pretty permalinks”, any Permalink setting under WordPress Settings | Permalinks other than “plain”.
The problem is that most of the Codex articles on the subject of doing a “WordPress in it’s own directory” installs came out well before the REST API existed. Most, dare we say ALL, have not been updated since and completely ignore the corner case of a WordPress subdirectory install with Permalinks enabled.