Resolution:
You might have run across this when you switched a registered Transformer from HTTP to HTTPS and then back to HTTP. Once you had done this, try to login by visiting the Transformer URL. It will throw up an error saying `Too Many Redirects`.
To test if it’s a browser problem, open a private/incognito session and try to log-in. If that works, then you need to go back to your normal window, go to Control Hub, and logout. A mismatched session cookie might be the culprit.