Occasionally placeholder [post_editlink] is empty, why?

The reason might be, that the user who creates the post status change does not have the right to edit the post. More likely is, that even no logged in user created the post status change, maybe if it was a schedule post. Because internally PSN uses the WP function "get_edit_post_link" to create the edit link and this function only works with a currently logged in user (see description here: https://codex.wordpress.org/Template_Tags/get_edit_post_link).

A solution would be to use template condition to check if the placeholder is empty and if so, show some other content:

{% if "[post_editlink]" is not empty %}[post_editlink]{% else %}[post_permalink]{% endif %}

Conditions have to be activated for this to work (http://docs.ifeelweb.de/post-status-notifier/conditional_templates.html)