At https://discuss-space.wmflabs.org/, clicking "Sign up" or "Log in" kicks the authetication with Phabricator correctly. With some versions of Chrome and Firefox, users can create their new account without problems. However, there are problems with at least some versions of Safari and Firefox.
Let's investigate. If you are experiencing problems, please list your browser version below.
If you are able to create and account and log in, please add your browser version too, so we can understand better the problem.