It is a common issue for users who are auto-capitalizing the first letter of their email address. I assume you are trying to reset the password from a mobile device? The email address field is case sensitive and any case variations will result in the "account doesn't exist..." error. Please be sure to use only lower case letters. If your issue persists, let me know.
Um, I believe that the RFCs state that the domain name must be case insensitive, but that the user name (to the left of the @) may be case sensitive or insensitive, an option for the server's domain.
Thanks for the responses! Restarting my hub worked - I'd never had this problem before and the hub's integration with Google Home has been reliable until now.
Embarrassingly, I actually knew about the case sensitive issue and I completely forgot about it!