ATT Yahoo Calendar wont sync since 18.0.1 update

There are other reports of Calendar problems in iOS 18.0 & 18.01 for both the iPhone and iPad. Here is but one of them: iOS 18 Calendar app hangs regularly with … - Apple Community

TLDR: on fix used by several users, myself included, is to switch calendar apps until the bug is fixed (one way to find out if the bug also applies to Yahoo is to try these steps):

Put your phone into airplane mode, ensuring that Wi-Fi is also turned off. Then restart your phone, go into control center, and remove your Yahoo accounts from calendar (I actually wound up removing all the calendar accounts). Next delete your calendar app (yes, really). Then download Microsoft Outlook for iOS (I know), and use that for your calendar until Apple fixes this.

I went so far to stop using and deleting Mail too, because I didn’t want to find out it would be buggy as well. This stopped the freezes and battery drains I experienced as I attempted piecemeal solutions.p (e.g. just removing calendar accounts and not the app itself).

DMarkR wrote:

The AT&T (Yahoo) email is my primary/default. Since the iOS update, only Gmail and iCloud populate on my iPhone. I did all the recommended steps: reboot (multiple times; delete and reinstall the AT&T/Yahoo email on my iphone, etc) Settings are set to fetch. When I pull up calendars on the iPhone, the image attached shows what I see. The warning sign (yellow triangle) is populated on the AT&T email account, but is not "active". Touching it gives no options. Any sug

https://discussions.apple.com/content/attachment/84fb0b0b-0649-48e9-bcc4-bc0fe963da92

Your next step is to report your issue to AT&T/Yahoo support.