Apparently it infers a username from your submitted email address. However, the username key is set as unique and errors out, dumping its debug info on the page:

<class 'psycopg2.IntegrityError'> at /register duplicate key value violates unique constraint "rcpts_user_user_login_key" DETAIL: Key (user_login)=(receiptapp) already exists. Python /usr/local/lib/python2.7/dist-packages/web/db.py in _db_execute, line 586 Web POST https://receiptapp.org/register