On this Web site: http://www.olspn.school-uk.org

When you click on View Calendar in the Side Bar.

And then Subscribe -> Add to Google+

You get:

Do you want to add this calendar?
Would you like to add calendar: http://www.olspn.school-uk.org/?plugin=all-in-one-event-calendar&controller=ai1ec_exporter_controller&action=export_events&no_html=true?
Yes, add this calendarNo, do not add this calendar
You click Yes and then you get:

Cannot add calendar “http://www.olspn.school-uk.org/?plugin=all-in-one-event-calendar&controller=ai1ec_exporter_controller&action=export_events&no_html=true”
You do not have access to http://www.olspn.school-uk.org/?plugin=all-in-one-event-calendar&controller=ai1ec_exporter_controller&action=export_events&no_html=true’s calendar
OK

Please can you advise what is wrong.