Moodle should check if the first recurring event exists before providing the option "Delete all" to the user. By modifying: file calendar/lib.php, function count_repeats() ... /** * Return the number of repeat events there are in this events series * * @return int */ public function count_repeats() { global $DB; if (!empty($this->properties->repeatid)) {   // verify if the event exists $event = $DB->get_record('event', array('id' => $this->properties->repeatid)); if($event) {   $this->properties->eventrepeats = $DB->count_records('event', array('repeatid'=>$this->properties->repeatid)); // We don't want to count ourselves $this->properties->eventrepeats--;   } } return $this->properties->eventrepeats; } OR calendar/delete.php, the section that check repeated events // If there are repeated events then add a Delete Repeated button $repeatspan = ''; if (!empty($event->eventrepeats) && $event->eventrepeats > 0) {     // verify if the event exists $repeatedevent = $DB->get_record('event', array('id' => $event->repeatid)); if($repeatedevent) {   $url = new moodle_url(CALENDAR_URL.'delete.php', array('id'=>$event->repeatid, 'confirm'=>true, 'repeats'=>true)); $buttons .= $OUTPUT->single_button($url, get_string('deleteall')); $repeatspan = '<br /><br /><span>'.get_string('youcandeleteallrepeats', 'calendar').'</span>';   } }

Test1 Add a repeated event to calendar (Site pages>calendar>new event) Goto the day in which the first event of the series appears Click on delete for the first event Make sure you are able to delete the event without any issue Goto the database table events and make sure the "repeatid" column for rest of the entries of the series points to next event of the series. Now visit any other event of the series and try deleting it. Make sure you can delete it without any issue. Try deleting the complete series and make sure you dont get any errors and events are deleted. Test it in more than one database. Test2 add following line to your config.php after the line in which $CFG->dirroot is defined $CFG->calendar = 'cal';; create a folder 'cal' in your calendar dir. copy the attached file to the just created folder. follow step 1-3 above make sure the script dies echoing the id of the next event in the series (next to the event just deleted) Note:- you might notice other un-related errors. See linked issues for further details.