Thanks for getting in touch with the Community, MCI.

Since multiple users are being informed their usernames don't exist while they're then still able to sign in after clearing cached data, I'd recommend getting in touch with our Customer Care team. It's possible there could be a connection issue on the backend with these particular usernames/emails.

You can get in touch with them using any of the options available on our Contact page. Also be sure to review their support hours so you'll know when agents are available.

I'll be here to help if there's any additional questions. Have a great day!