How do I fix Apple Watch login problems after changing Apple Account email address?

If you have an older type like version 3, it might help to change your password in something simple, eight digits with 7 charactrs and 1 number. Had the same problem. but after a lot of password changes, this worked. After this, you can set it back to something more complex.

Rethinking this, and seen the date of occurrence, it might also have been a problem at the server site ;-)

When changing the email associated with your Apple ID, it can sometimes lead to login issues on devices like the Apple Watch. I recommend trying to reset the watch again, ensuring it's connected to your iPhone during the process to sync the updated account information. If the problem persists, contacting Apple Support for further assistance would be beneficial.