Resolved
Overview of the issue
If you translate the Portolio base category provided by Avada theme, it may lead to 404 errors on the front-end.
Workaround
Please, make sure of having a full backup of your site before proceeding.
- Open …/wp-content/themes/Avada/includes/lib/inc/class-fusion-multilingual.php file.
- Comment out this line
//add_filter( 'option_rewrite_rules', [ $this, 'wpml_portfolio_slug_filter_rewrite_rules' ], 1, 1 );