I signed up, activated the account using the email link, then tried to sign in:

If I reload the page entirely (the 500 error goes away) and sign in again, it says wrong username/password combination (I autofill username and password using my password manager, so no mistakes there). What's more interesting is that it allows me to sign up again using the same username, even though the account should exist after activation.
NB: I was previously able to sign up without any trouble using my personal email address.