Since upgrading to 7.5.6, existing calendars on my sites don't do anything when clicking on the ical link, and the log gives:

2008/03/19 00:04:11 - WARN - www.site.com.conf - main::[[undef]] - Couldn't call method ical on as
set for url: calendar/calendar Root cause: The 'seconds' parameter (undef)
to DateTime::Duration::new was an 'undef', which is not one of the allowed types: scalar
 at /data/wre/prereqs/lib/perl5/site_perl/5.8.8/i686-linux/DateTime.pm line 1316

Adding a new calendar to the site is ok, its ical link works.