Recently iCal export for 1 out of my 3 calendars stopped working on my Owncloud installation.
Turned out it was a defective entry. This entry missed the correct closing tags %END:VCALENDAR%.
I used the following SQL statement to find all entries missing the correct closing tags.
SELECT * FROM oc_clndr_objects WHERE calendardata NOT LIKE ‘%END:VCALENDAR%’;
Once they were removed, iCal export was working as expected for the calendar in question.