iOS 10 icloud calendars not showing up (gone after upgrade) After upgrading my iPhone 5 from iOS 9 directly to iOS 10.0.1, all my icloud calendars are gone.

They don't show up in the calendars list in the Calendar app, and dragging said list down to force syncing of the calendars doesn't even show the usual spinning loading bar indicator – the whole iCloud section is just missing from the list. Rebooting didn't fix the issue.
The following finally fixed the issue for me:


Sign out of iCloud (Settings -> iCloud -> Sign Out)
choosing "delete all iCloud data from iPhone"
sign in again
wait a couple of minutes