Hi. I have hit the same problem and have no idea when it happened either.
Having read your post and many others I thought I'd try the fix involving turning off reminders synchronisation. What I did was typed the reminders into my iphone, quit reminders on the mac, turn off synchronisation on the mac deleting all entries on mac, log into iCloud using safari, noticed all reminders present, rebooted mac, turned on synchronisation merging entries, opened reminders and all were there.
It may be the iPhone and safari bits were not needed and only the reboot, or it may be we have a different fault. Anyway thought I'd share what worked.
I have developed the same problem. Calendar and Reminders no longer sync, nor can I enter any new items via iCloud.com. My iPhone and BusyCal on my MAC no longer sync. I was tempted to try turning sync off on my MAC but, like you, got the warning that it was deleting all data on my MAC. So I backed off.
Using my MAC, I signed onto my wife's iCloud account and experienced no problem; all works as it should.
I keep sending the error messages to Apple. So far in the last two days they have not responded.
I came on here looking for an answer to the same problem but I've sinced solved it.
I had iCloud set as my default calendar on the phone and it worked fine. Then I found that Phone had stopped syncing the calendar. I checked in the settings and found that my default calendar had changed (I don't know why but possibly it was an update). I reset the default calendar to iCloud and now it's working fine again.