I think I just tripped over this, and found my answer in a random support post elsewhere.
I created an account and then was unable to log in as it, the Login button would activate, turn into a spinner, and then revert to 'login'.
It turns out that the instance was waiting for me to validate my email address. This may be a larger Lemmy problem instead of an instance one, but having the login button identify that it failed due to incomplete account setup (missing email validation) would be helpful for end-users.