I tried registering with e-mail address ***@inbox.com. I got the error:
Why? This is a pretty private paid e-mail service provider; why is this e-mail not allowed?
Next, I entered a work e-mail. I get an error,
Captcha not verified. Are you sure you're not a robot?
I verified Captcha the first time I tried signing up. The second and subsequent times it simply displays a checkmark showing "Captcha verified", and does not ask me to verify another captcha. Somehow the server needs to recognize that the captcha has already been verified.
Re-starting the browser session allowed me to verify another captcha, which solved the problem.