Ok, but first try these steps:
1. Put https://website.com/my-inbox/?pmaction=newmessage&to=1
in any post (adjust where needed for url and the page where you have put [cartpauj-pm]
). If that link brings up the message template to be sent to the admin of the WordPress installation generated by Cartpauj, then the above step in explained should work in EVERY post. If however you already get here an issue with 404 error message, then there is something wrong with your Cartpauj instance itself.
2. In case an error occurs, put [content field="author" meta="ID"]
in an article post. What do you get, do you get an ID number? If you do this in a post you wrote as admin, then you should get the number 1. If not (no number shown or simply shows the code itself, then you have an issue with the “Custom Content Shortcode” plugin.