Brajesh,

I have contacted the developer and will let you know as soon as I get their replay. In the mean time I restricted the usernames by inserting the following code into functions.php

function wpsnippet_validate_username($valid, $username) {
    $restricted = array('profile', 'directory', 'domain', 'download', 'downloads', 'edit', 'editor', 'email', 'ecommerce', 'forum', 'forums', 'favorite', 'feedback', 'follow', 'files', 'gadget', 'gadgets', 'games', 'guest', 'group', 'groups', 'homepage', 'hosting', 'hostname', 'httpd', 'https', 'information', 'image', 'images', 'index', 'invite', 'intranet', 'indice', 'iphone', 'javascript', 'knowledgebase', 'lists','websites', 'webmaster', 'workshop', 'yourname', 'yourusername', 'yoursite', 'yourdomain', 'mod');
    $pages = get_pages();
    foreach ($pages as $page) {
        $restricted[] = $page->post_name;
    }
    if(!$valid || is_user_logged_in() && current_user_can('create_users') ) return $valid;
    $username = strtolower($username);
    if ($valid && strpos( $username, ' ' ) !== false) $valid=false;
    if ($valid && in_array( $username, $restricted )) $valid=false;
    if ($valid && strlen($username) < 5) $valid=false;
    return $valid;
}
add_filter('validate_username', 'wpsnippet_validate_username', 10, 2);
 
function wpsnippet_registration_errors($errors) {
    if ( isset( $errors->errors['invalid_username'] ) )
        $errors->errors['invalid_username'][0] = __( 'ERROR: Invalid username.', 'wpsnippet' );
    return $errors;
}
add_filter('registration_errors', 'wpsnippet_registration_errors');

Taken from:
https://wp-snippet.com/snippets/how-to-restrict-usernames-in-wordpress-without-a-plugin/

However, the same issue happens where a blacklisted username (mod) first shows as available: https://prnt.sc/nrpy0p,
then it shows it as being blocked AFTER I click register: http://prntscr.com/nrpygr

Could you kindly provide some guidance as to why this happens?
Thank you so much.

Brajesh,
Thank you so much for helping me with this.

Brajesh,

I am not using it on multisite. This is what I see upon typing the word ‘image’ it in: http://prntscr.com/nshmb1
This is what happens after I press ‘register’: http://prntscr.com/nshnsy
And upon typing the word image again, this happens: http://prntscr.com/nsho2e

I am quite unsure as to why this is happening.

Thank you!

P.S: I originally had modified the code to allow a minimum of 3 character letters but I changed it back to see if that could be causing the error. Unfortunately, the error still happened.