I tried clearing my Memmy for Lemmy (iOS) cache but I still couldn’t log in. Removed and reinstalled the app (no trouble) and I can log in again. (Just had to re-do my preferences - again, just a minute or so).

Just to add to this.

This is because the authentication tokens that your browser uses are stored in cookies, these match up with entries in the database.

As a way to mitigate the hack, admins deleted the entries in the database rendering the tokens in cookies useless.

This means that any tokens the hacker got access to are also useless.