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You're right again. Horde 5 passes the default options to libcurl as 
the command-line curl which means self-signed certs cannot be 
verified, is this correct?

So to solve this the correct way, I'd need to download the SSL 
certificate of the calendar webserver, and put the cert to eg. 
/etc/ssl/certs, correct?

I'm just trying to understand. But maybe this changed behaviour should 
be documented somewhere.

It doesn't have anything to do with Horde either. Fix you server to 
properly verify SSL certificates.

You're right, it has nothing to do with Kronolith, but with Horde. I 
changed verifyPeer to "false" in 
/usr/share/php/Horde/Http/Request/Base.php, then ran 
"horde-clear-cache", restarted Apache (just to be sure) and now the 
remote calendars show up.

SSL certificates or remote calendars haven't been verified in Horde 4, 
now they are. And your server cannot verify them, which has nothing to 
do with Kronolith.

When subscribing to remote ICal calendars, the following error popup 
is returned when you activate the checkbox to display that remote 
calendar:

Zimbra error:
Problem with 
https://mail.example.org/service/user/username@example.org/Calendar: 
fopen(): SSL operation failed with code 1. OpenSSL Error messages: 
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate 
verify failed. fopen(): Failed to enable crypto. 
fopen(https://...@example.org/Calendar): failed to open stream: 
operation failed

Google Calendar error:
Problem with 
https://www.google.com/calendar/ical/user%40gmail.com/private-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/basic.ics: fopen(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed. fopen(): Failed to enable crypto. fopen(https://www.google.com/calendar/ical/user%40gmail.com/private-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/basic.ics): failed to open stream: operation 
failed

This has been tested with:
- Google unauthenticated ICal, valid certificate (I suppose)
- Zimbra authenticated ICal, self-signed certificate
- Both with HTTPS URLs

Server OS: Debian stable (squeeze) 64bit

This worked in Horde 4, so I suspect the error could even be in a PEAR 
package.