That sounds like a bug to me; the registration should give an error as long as it’s not filled in. I’ve made an issue about that. (Wow, we’re over 400 issues…)
So, there’s apparently an issue for some people that they cannot paste it in one go. I’m not sure why this is, but that’s certainly the reason why the registration does not work for you. There should be no line breaks in the whole string, not even leading nor trailing.
I’ve had someone suggest on Matrix to use something like vi a; harbour-whisperfish --verbose --captcha "$(cat a)"
, such that you can cleanly edit it in vi
(replace with nano if you want) and then on close automatically start Whisperfish. Would that help?