Calendar asking password to use after iOS 18 update

There could be a variety of reasons behind the issue. Here are some steps you can try to troubleshoot and potentially resolve the syncing problem:

1. Verify if the private CalDAV service you are using is experiencing any downtime or issues. Sometimes, server-side problems can cause syncing to fail.
2. Ensure that your iPhone has a stable internet connection. Syncing relies on a reliable network connection, so make sure you are connected to Wi-Fi or cellular data.
3. Double-check the account settings for your CalDAV account. Make sure the server address, username, password, and other settings are correct. It's easy to overlook a small mistake that can prevent syncing.
4. Some CalDAV servers require SSL/TLS certificates for secure communication. If the server's SSL certificate has expired or there's an issue with it, syncing may fail. Verify the certificate's validity and update it if needed.
5. If you're using a firewall or security software on your iPhone, it might be blocking the connection to the CalDAV server. Check the settings of any security software and ensure that the CalDAV service is allowed.
6. If you're using a third-party app for CalDAV syncing,
   1. Make sure the app has the necessary permissions to access your account and data. This includes calendar access and background syncing.
   2. Check if there are any updates available for the app. Updated versions might have bug fixes and improvements related to syncing.
7. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
8. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
9. Though you have perceived that the issue was due to an iOS update I want to emphasize the importance of making sure you have the most recent updates installed. This step can play a crucial role in troubleshooting and resolving the issue.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.

iPhone Calendar? Apple Calendar? There is no such thing.

Please check which is your Default email Account for the Calendar on your device? Settings > Calendar > Default Calendar

All the devices sync with an email client server which hosts your calendar. If you have a Calendar on iCloud that will not sync with Google Calendar. Settings > Calendar > Default Calendar [Choose what you want]. What you add (Event) on iPhone Calendar is synced with default calendar unless you have chosen a specific calendar at the time of event creation.

To which email account are the calendar events synced with your iPhone? This is not clear in your post

See the iPhone and Mac Calendar Pics below

Which is your default calendar? Settings > Calendar > Default Calendar

Settings > Calendar > Sync > [All Events] - Watch the GIF below

It's not always the iOS update's fault!  Many things can cause phone problems.  If you tell people you think it's an iOS update, it might make it harder to fix or find a solution for the real issue.

Remember, the right question often leads to the solution!

iOS updates aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. The majority of the issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors. This could be a pure coincidence that the iOS update affected the iPhone functions.

Before initiating a standard iOS update, it's advisable to create a backup. Have you completed this step? If yes, proceed with the restoration from the backup.

See what each update does: About iOS 18 Updates, About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...