WordPress admin_post not working

Hi

I have the following code for my plugin. However it works as it should when I am logged in but when I am not logged in submitting the form takes me to a WP admin “Not Available” screen.

How come and what can I do to fix this?

<?php
    /*
    Plugin Name: Subscriber Check
    Version: 1.0
    Author: Shivam Paw
    Author URI: https://www.shivampaw.com
    Text Domain: subscriber-check
    Domain Path: /languages
    Description: Plugin that means a viewer must enter the email address for mailpoet to see content.
    */

    if (!session_id()) {
        session_start();
    }
    function subscriber_check_shortcode( $atts, $content = null ) {
    	if($_SESSION["subscriber_check"] == true){
    		return $content;
    	}else{
    		$message = "<p>Please enter the email you used when subscribing to our site to view this page. Once verified you will be able to access this page until you close your browser.</p>";

    		$error = "";
    		if( basename($_SERVER['REQUEST_URI']) == "?error"){
    			$error .= "<p style='color:red;'>The email you entered was not found. Please make sure you have confirmed your subscription. You must enter the email address you used when subscribing. If you can't remember the email then please re-subscribe.</p>";
    		}

    		$form = '
    		<form method="post" action="'.get_admin_url().'admin-post.php" id="subscriber_check_form">
    			<p>
    				<input type="hidden" name="action" value="submit-form-subscriber">
    				<input type="email" name="subscriber_check_email" id="subscriber_check_email" placeholder="Enter Your Email">
    				<input type="submit" name="subscriber_check_submit" id="subscriber_check_submit" value="Verify">
    			</p>
    		</form>
    		';
    		return $message.$error.$form;
    	}
    }
    add_shortcode( 'subscriber_check', 'subscriber_check_shortcode' );

    add_action('admin_post_submit-form-subscriber', '_handle_form_action'); // If the user is logged in
    add_action('admin_post_nopriv_submit-form-subscriber', '_handle_form_action'); // If the user in not logged in

    function _handle_form_action(){
    	if($_POST["subscriber_check_email"]){
    		global $wpdb;
    		$sql = $wpdb->prepare( "SELECT * FROM 2proiTFuxs_wysija_user WHERE email=%s AND confirmed_at IS NOT NULL",$_POST["subscriber_check_email"]);
    		$results = $wpdb->get_results( $sql);
    		$rows = $wpdb->num_rows;
    		if($rows != 0){
    			$_SESSION["subscriber_check"] = true;
    			header('Location: /free-downloads');
    		}else{
    			header('Location: /free-downloads/?error');
    		}
    	}
    }