This task took so long to finish that at first I commented out the refresh lines. That certainly kept the events from refreshing, but it allowed the cron job to complete.
lib\classes\task\refresh_mod_calendar_events_task.php
//
Refresh events.
// mtrace('Refreshing events for ' . $plugin->name);
// course_module_bulk_update_calendar_events($plugin->name, $courseid);
Next, I decided to run the task again, but uncommented the lines to see if the refresh would get hung up again.
To my surprise everything refreshed successfully in 15 seconds with 250 queries run.
This is not the real fix for this issue, but could be a temporary work around.
Execute adhoc task: core\task\refresh_mod_calendar_events_task
... started 14:33:03. Current memory use 105.3MB.
Refreshing events for assign
Refreshing events for assignment
Refreshing events for book
Refreshing events for certificate
Refreshing events for chat
Refreshing events for checklist
Refreshing events for choice
Refreshing events for choicegroup
Refreshing events for data
Refreshing events for edivate
Refreshing events for feedback
Refreshing events for folder
Refreshing events for forum
Refreshing events for game
Refreshing events for glossary
Refreshing events for hotpot
Refreshing events for hvp
Refreshing events for imscp
Refreshing events for journal
Refreshing events for label
Refreshing events for lesson
Refreshing events for lti
Refreshing events for mylabmastering
Refreshing events for oublog
Refreshing events for page
Refreshing events for pcast
Refreshing events for pdthreesixty
Refreshing events for questionnaire
Refreshing events for quiz
Refreshing events for resource
Refreshing events for scorm
Refreshing events for survey
Refreshing events for turnitintool
Refreshing events for turnitintooltwo
Refreshing events for tutorialbooking
Refreshing events for url
Refreshing events for wiki
Refreshing events for workshop
Refreshing events for lamslesson
Refreshing events for slideshow
... used 250 dbqueries
... used 15.125571966171 seconds
Adhoc task complete: core\task\refresh_mod_calendar_events_task