This error means that cURL’s CA cert bundle on your server is out of date.
To fix this problem, update cURL’s SSL certificates on your server. Contact your web hosting provider with the request. Read more at Details on Server SSL Certificates.
Alternately, you can manually edit ThinkUp’s application set cURL to not verify HTTPS connections. Anywhere ThinkUp connects to an SSL URL, add the following lines before the curl_exec call:
$opts[CURLOPT_SSL_VERIFYHOST] = false;
$opts[CURLOPT_SSL_VERIFYPEER] = false;
See also this mailing list thread about SSL certs.