FWP creates users with empty usernames if username does not contain alphanumeric characters
I've found this solution: Install Transliterado and use this code:
if(function_exists(sanitize_title_transliterado))
{
$this->post['named']['author'] = apply_filters('syndicated_item_author', sanitize_title_transliterado($this->author()), $this);
}
else
{
$this->post['named']['author'] = apply_filters('syndicated_item_author', $this->author(), $this);
}
near line 50 in syndicatedpost.class.php
I'd prefer FWP be more international in this case