It seems I have the same issue like mentioned here: not-a-link://devforum.okta.com/t/unable-to-log-in-due-to-403-error/29437/1. My org is dev-25927875.
I created a test dev account but used my GitHub to signup. It seems that user credentials then are somewhat invalid to the default policy. I was only able to login via that welcome/password change link in the email. After signout/expiry of that link you cannot login again as admin.
I suspect it’s related to this: not-a-link://support.okta.com/help/s/article/Okta-admin-is-unable-to-log-into-the-Admin-console-and-getting-an-error-Sign-in-Denied?language=en_US. But then you cannot ask another admin (I’m the only initial user) or create a support ticket (requires login).
So besides unlocking my account I’d suggest to adjust the default new account policy to work also with GitHub credentials. Or have the email share instructions on how to configure the account for permanent use.
(note that it’s really awkward to come here, create a forum account, receive warnings that I cannot add links just to submit a support case - luckily devforum doesn’t require Okta login :))