Wordpress 4 invalid username special charachters issue

Found the answer, created the following function:

function wscu_sanitize_user ($username, $raw_username, $strict) {
    $username = wp_strip_all_tags ($raw_username);
    return $username;
}
add_filter ('sanitize_user', 'wscu_sanitize_user', 10, 3);

Are you creating your users via regular means or somehow importing them?

Trying to create such user WP prevents me from doing so:

ERROR: This username is invalid because it uses illegal characters. Please enter a valid username.

The reason for that is that username is validated with validate_username(), which runs sanitize_user() in strict mode. That reduces available characters to basic ASCII set.

In a nutshell WP doesn't consider that a valid username.

So the question is — is that actually a username in your case? Your users might be confusing Username with First/Last name fields, which are not used for login.

If you do want to support usernames like that you will have to override WP validation via hooks. That would probably take some serious testing to make sure it works properly and doesn't introduce security issues.