You could run the ics through a few validators in case you have not:
http://severinghaus.org/projects/icv/
http://icalvalid.cloudapp.net/
http://mozilla-comm.github.io/ical.js/validator.html
In Lightning, make sure you are using iCalendar / WebDAV when subscribing to the calendar. You can also use the advanced config editor (Options -> Advanced -> General -> Config Editor
) to enable the preferences calendar.debug.log
and calendar.debug.log.verbose
.