Redirected to “Redirection url” (404) after frontend-login

Hi!
On my website I have a frontend-login (selfmade). But after the users log in via that, they can’t access the backend via the WPS Hide Login Login URL (or any other url) – they get redirected to 404, which is set at Options > “Redirection url”.
I made the login just as wordpress recommends it, you can see my code below. (Except an ajax function is doing the login, which shouldn’t make a difference!)

I found out, which codepart of wps hide login acts odd:

if ( is_admin() && ! is_user_logged_in() && ! defined( 'DOING_AJAX' ) && $pagenow !== 'admin-post.php' && $request['path'] !== '/wp-admin/options.php' ) {
   wp_safe_redirect( $this->new_redirect_url() );
   die();
}

This is the codepart which redirects me to 404, even though the user is logged in (I also printed out is_user_logged_in() and it returns true.)

This is my code:

add_action( 'wp_ajax_nopriv_logInUser', 'ii_ajax_log_in_user' );
add_action( 'wp_ajax_logInUser', 'ii_ajax_log_in_user' );

function ii_ajax_log_in_user(){ 
        $username = sanitize_text_field($_POST['email']);
        $password = $_POST['password'];
        $remember = $_POST['remember'];
        
        $login_data = array();
        $login_data['user_login'] = $username;
        $login_data['user_password'] = $password;
        if($remember) {$login_data['remember'] = true;}
        
        $user_verify = wp_signon( $login_data, false ); 

}