[horde] Problem with "Check for newer versions"

[Matze]

Hi everyone,

i have set up a new Horde Installation which works well except:

I experience a weird Problem when checking for newer versions via the menu:

Administration -> Configuration
"Check for newer versions"

I get an error message popping up, saying:

"Could not contact server. Try again later."

A tcpdump showed, that there is probably a problem with "unknown ca" while connecting to https://ftp.horde.org.

Horde Debug Log doesn't show any error messages.

Mine is a Ubuntu server which has the "ISRG Root CA" in /etc/ssl/certs

But maybe horde uses another Source for validating?

Is there a better way to debug that issue?

- Horde installed via PEAR
   o Version Groupware 5.2.22
- Ubuntu 14.04.5 LTS
- Apache 2.4.7
- PHP 5.5.9

Regards,

Matze