Craft Commerce 2: PayPal Express not working - Correlation id: unknown
The Craft support helped me to figure this out. The problem was, that the CSRF wasn't properly disabled.

So in my config/general.php I have the following:

if ( !function_exists('isApiRequest') ) {
    function isApiRequest ()
    {
        $path = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
        $query = parse_url($_SERVER['REQUEST_URI'], PHP_URL_QUERY);

        $actions = [
            'actions/charge/webhook/callback',
            'actions/commerce/payments/complete-payment',

        ];

        foreach ($actions as $action) {
            if ( strpos($path, $action) || strpos($query, $action) ) {
                return true;
            }
        }

        return false;
    }
}

return [

'*' =&gt; [

    'enableCsrfProtection' =&gt; !isApiRequest(),
    ...