Ok, thanks for investigating. I've been careful not to modify core files, instead, I created a child theme and only modified styles.css, the header file, the footer file, the single.php file.
This is the only new function I added to functions.php
function add_categories_to_pages()
{
register_taxonomy_for_object_type('category', 'page');
}
add_action('init', 'add_categories_to_pages');
function add_tags_to_pages()
{
register_taxonomy_for_object_type('post_tag', 'page');
}
add_action('init', 'add_tags_to_pages');
add_post_type_support('page', 'excerpt');
I've activated the main WPLMS them instead of the child theme and the issue remains.
I've downloaded the .zip from Themeforest and uploaded and activated, still the problem remains. I don't know what else I can do?