[horde] Horde install using composer fails

[Horst Simon]

Hi,
I am trying to install Horde 6 using composer on a QNAP NAS. when I run composer install I am getting  following error:
Loading composer repositories with package information

In CurlDownloader.php line 365:
                                                                               
  curl error 60 while downloading https://horde-satis.maintaina.com/packages.  
  json: SSL certificate problem: certificate has expired                       

Which certificate is expired? Is it on my site or on the server?

Regards,
Horst