Q: Why has iCloud stopped syncing calendar changes and reminders on my Mac with my iPhone?

I'm pleased to report the fix outlined by Randers4 works. Here's an edited version of his instructions which I followed:

Sign out of iCloud on your iPhone/iPad and sign back in:

Go to Settings>iCloud, tap Sign Out, choose Delete from My iPhone/iPad and provide your password when prompted (your data will still be in iCloud).  Then reset your phone (hold the power and home buttons at the same time until you see the Apple logo, then release); when it restarts go back to Settings>iCloud and sign back in (your data will download back to your phone.  Then try syncing to your Mac (and vice versa) again and see if it works properly now.

I'm now a happy bunny!

Hi sanders4,

After rebooting my iPhone and logging in again in iCloud worked for me.

Thanks!

Dennis

For what it's worth, I had similar symptoms, but a different- self inflicted- cause. I had acquired over the years two Apple ID's, one associated with a .mac address that we had used as a "family" email, another (based on our company domain/ email) used for personal and business. Since having two Apple ID's has become problematic to say the least, when I upgraded to a new Mac and phone I tried to consolidate and use just one for all iCloud functions, leaving the other .mac address just for email. I was able to do this fine for everything except Calendar. The only "fix" I found was to disable the legacy iCloud account in Calendar preferences, and re-enter everything under the iCloud account I was using as my Main one. iCloud can only see one Apple ID, and I had my calendar data somewhere it could not look.