IPN侦听器突然停止工作

我的贝宝IPN监听器已经工作了一年多了，今天它突然停止工作，出现错误

cURL error: [60] SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

我的卷曲设置是

protected function curlPost($encoded_data) {
    if ($this->use_ssl) {
        $uri = 'https://'.$this->getPaypalHost().'/cgi-bin/webscr';
        $this->post_uri = $uri;
    } else {
        $uri = 'http://'.$this->getPaypalHost().'/cgi-bin/webscr';
        $this->post_uri = $uri;
    }
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
    curl_setopt($ch, CURLOPT_CAINFO, 
                dirname(__FILE__)."/cert/api_cert_chain.crt");
    curl_setopt($ch, CURLOPT_URL, $uri);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $encoded_data);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, $this->follow_location);
    curl_setopt($ch, CURLOPT_TIMEOUT, $this->timeout);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HEADER, true);

    $this->response = curl_exec($ch);
    $this->response_status = strval(curl_getinfo($ch, CURLINFO_HTTP_CODE));
    if ($this->response === false || $this->response_status == '0') {
        $errno = curl_errno($ch);
        $errstr = curl_error($ch);
        throw new Exception("cURL error: [$errno] $errstr");
    }
}

我尝试使用http://curl.haxx.se/ca/cacert.pem的最新证书作为api_cert_chain.crt，正如许多相关答案中所建议的那样，但它没有改变任何事情。

这可能是什么原因造成的？