Can't sync calendars with iCloud I tried to enable Calendar sync at Settings > iCloud > Calendar and a message says "Activating Calendar sync" but it ends up in Home screen with no changes. When I go again to iCloud setting I see Calendar option is not enabled.

Another problem I found is that I can not invite other people to appointments. It seems iCloud is not working for my Calendar.

How can I enable iCloud Calendar syncing?
Try removing the iCloud account from your iPhone, and then adding it back. This has solved a number of my own iCloud-related issues in the past.