Calendar could not be synced or exported

All of a sudden I could not sync one of my calendars with my iPhone or Thunderbird any more. Since all my other calendars would sync as normal, it probably could not be a problem of my server configuration. While the calendar was shown in the web interface properly, I discovered that I could not export the calendar as well.

Then I found this:
Indeed, my database contained one entry with invalid data which caused all the trouble.
Using the following MySQL query, I could detect the broken event:

SELECT * FROM oc_clndr_objects WHERE calendardata NOT LIKE ‘%END:VCALENDAR%’;

After deleting the database record containing the invalid data everything worked fine again.