Like so many other users, I have suffered with this issue. With the previous iOS update, we were able to fix it by deleting and reinstalling the account. With the last one, not so fortunate. The removal and re-entry of the password did nothing, either. I was just about to try going back to CalDAV when I thought to investigate something in my Yahoo! account on my laptop, and it worked. And it was easy. And now my calendar is properly syncing across the Web, my iPhone, and iPad.
I went into my calendar on my laptop. On the left sidebar, under the calendar image, it says Calendars. If nothing is under it, click it to show your calendar/s name/s.
Make sure not to unclick the box to the left of the name of your calendar! Instead, click to the right of it.
Select Edit Calendar.
In the window that pops up, you'll see Share with iCal Address. In the field below it is your current iCal address. And, in the lower right corner of that field is a little refresh button. Click it. You've just reset your iCal address.
For good measure, and out of desperation, there were a number of other things I tried, too, at that point. I reset the web address under it the same way, and I changed my calendar name. (Yahoo! is fond of throwing errors when you make these changes. Just keep trying. It's least frustrating if you try to change just one thing at a time.)
At this point you really want to make sure your calendar has the same name on all of your devices. It should. If not, change it by going into the calendar on your device, selecting Calendars at the bottom of the display, clicking the information "I" icon to the right of your calendar's name, and renaming it at the top of the next screen.
If that doesn't work, try deletion and rentry again. If you do that, however, you will lose any unsynced events, which *****. After doing that, always make sure to go back and set Yahoo! as your calendar default (and default for anythign else, like contacts, etc.) in Settings on your device.
This worked for me when nothing else did. Perhaps it will work for you. Maybe it won't. Yahoo! and Apple seem to be a match made in ****. I'm puting this out there as one more thing to try. Just like all the other fixes, I expect mixed success with it.
