The default login/registration form of Open edX has a bug that will disable the submit button after the first fail attempt and seem like no one want to fix it. Fortunately, you can use the combined login/registration form which does not have that bug. You enable it by enable the following feature flag:
If you're using a custom comprehensive theme, you need to set it as a Site Configuration flag in the Django Admin.
But if you're not using a custom theme, you can just set it in your /edx/app/edxapp/lms.env.json file, under the FEATURES list.
Then restart your apps and enjoy:
$ /edx/bin/supervisorctl restart all