Given the object of the project, that may well be part of the puzzle that you have to solve.

I initially had same problem with the attached twice when I used httpS. AND then trying to reattach using http.

Solution was to detach, stop all BOINC tasks, exit BOINC, waiting about two minutes, restart BOINC, resume tasks and finally attached to the project using the http address.

Regarding the couldn't create account when trying to join using the BOINC program... if you would of read the front/main page of the project it specifically stated that you need a 10 character password AND must sign up via the website for security reasons.

*** Edit to add the "Attached Twice to project" error message is very confusing to the novice. I believe it's root is from when BOINC client and/or server code was recently changed to automatically re/attach using https. It should be looked at for clarifying and/or modification by the BOINC code maintainers.