Hi,
This one was a though one to figure out, but I found a solution.
After debugging for some time I realized that the page where the plugin shortcode is placed is not actually a post type object (post, page etc..) but a custom page displayed via the slug – so there is no global post object which Related Posts Pro could use as a reference.
This means that the plugin is not able to get the current post (as it does not exist) and check for related items, and immediately exits. Since now I have not thought about such usage of the plugin, so it was not able to handle it.
I’ve made a few changes in the plugin configuration, as well as in the plugin code to try getting random items from the database when no post content is found. It’s only a temporary solution for now, I will note this one as a possible bug, and I will come up with a fix for the upcoming release.