Password works in webmail but not when connecting to imap.comcast.net

You will need to have Third Party Access Security checked in your settings to allow 3rd party clients (outlook, thunderbird, etc) to access your comcast email. Adding this for clarification

That just means any changes to your mail on one device syncs on all connected devices, plus on comcast's email site. I did also hear other folks had issues with IOS 18 and just comcast too, so you're not alone

I would check this:
https://www.xfinity.com/support/articles/third-party-email-access

• Sign in to Xfinity Email using your Xfinity ID and password.
• In the top-right corner, click the Gear icon, then select Settings.
• Click Security.
• Uncheck the box under Third Party Access Security to prevent third-party programs access to your Xfinity Email, or check the box to allow access.

Also this might help: https://www.xfinity.com/mobile/support/device-troubleshooting?page=device/apple/iphone-xs-max-ios-18/topic/e-mail/setup-email/choice/xfinity-imap