Edit (14-Feb-2021): This has recently been resolved. The following is an extract of the solution I received in an email.
We got an update from the developer team.
Because of the initial mix-up of the WordPress.com accounts, it would be necessary to reset some metadata for the Secure Sign-On feature.
To reset the SSO security metadata, you will need to run the following database query:
delete from wp_usermeta where meta_key = 'wpcom_user_id' AND user_id = 1;
Before making any changes to the database, we strongly recommend creating a backup to be on the safe side.
Your host should be having a tutorial on how to do this as well, for example:
You can check out that as well.
After running this MySQL query, you should be able to log in via SSO using your account.
Reference: Jetpack forum thread Unable to login using WordPress.com
Jetpack WordPress.com login setting for site www.readymcgetty.com.au.
Login via WordPress.com login fails, but login using Username and Password is ok. Preference is to use the former. Refer to the embedded video, which demonstrates the issue.
Clarifying video that shows login error is with the branded email rather than gmail.