Get the latest release of private GitHub repository for theme update

I wanna get the latest release of my theme which is stored in a private repository using pre_set_site_transient_update_themes but I failed with the download link and the authentication.

function github_fetch_user_data($githubArray)
{   
    $token = '<token>';
    $url = 'https://api.github.com/repos/<user>/<repo>/releases';
    $args = array(
        'headers' => array(
            'Authorization' => 'Basic ' . base64_encode('<user>' . ':' . $token)
        )
    );
    $response = wp_remote_get($url , $args);
    $body     = wp_remote_retrieve_body($response);
    $api_response = json_decode($body, true);
    //get array of latest release
    $api_detect =   $api_response[0];
    $githubArrayPart = $api_detect[$githubArray];
    
    return $githubArrayPart;
}

I use this function to get a API request but when I use the zipball url with github_fetch_user_data('zipball_url'); with wordpress turns into maintanance and nothing more happens.

What is wrong with my release link or is it impossible to download a privat repository in this way?