Problem:

Unable to access a specific page in WordPress Admin console (post type: chapter) after upgrading the server to PHP version 8.0 or above. The error points to a custom-written plugin causing an "E_ERROR" in class.wpcf-post-types.php.

Solution:

Identified a custom-written plugin (Love on a Leash Members) causing the conflict, particularly with the usage of toolset_get_related_posts() and toolset_connect_posts() functions. The custom plugin needs to be updated to use the current implementation of these functions as outlined in Toolset documentation.

Relevant Documentation:

https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/#toolset_get_related_posts

https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/#toolset_get_related_posts-legacy