Problem in adding new site on WP multisite

The problem was solved in the next update of WordPress. By the way if you still haven’t updated your version of WordPress:

It’s enough to set DB_COLLATE in the wp-config.php:

define('DB_COLLATE', 'utf8mb4_general_ci'); // on condition that your DB_CHARSET is 'utf8mb4'

That’s it!

The bug is hidden in the file \wp-includes\wp-db.php on the line 731 in the function init_charset(). For multisite mode the ‘utf8_general_ci’ collation is used if DB_COLLATE is empty. And if your DB_CHARSET is ‘utf8mb4’ then the error occures.

The actual error is raised when creating and consequently querying nonexistent tables for the new site (‘2’ changes with the number of sites):

wp_2_terms

wp_2_term_taxonomy

wp_2_term_relationships

wp_2_commentmeta

wp_2_comments

wp_2_links

wp_2_options

wp_2_postmeta

wp_2_posts

someone suggested this solution and it works fine.