I had also an issue with the default calendar module on an iOS 9.3.5 iPad. It could not be rendered. On debugging the iPad Safari with my MacBook i found the reason here: modules/default/calendar/calendar.js, Line 439.

for(let evt of eventList){

should be

for(var evt of eventList){

so that older browsers can render the module without error. Would be great if you can add that for the next release :folded_hands: