Dear Mike,
updates and extra configurations done considering your site installation and the way the users information is available:
1. update Access configuration and settings to the 'Message' custom post type
2. edited the 'My Messages' page
[wpv-view name="My Messages received" info="[wpv-current-user info="user_nicename"]"]
this way we are filtering the messages based on the user_nicename instead of the user ID
3. you need to update the classifieds_cred_save_data_new_message_function() in the classifieds.message_system.class.php file in order to reflect this when checking the 'message-to' custom field
add_post_meta($post_id, 'wpcf-message-to', $advertiser_data->user_nicename, true);
Could you test with a new user and confirm that you can now send and receive and view the messages in the My Messages page.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Ana