As reported by many confused users at http://git-annex.branchable.com/bugs/impossible_to_login_to_the_website_at_times/ a successful login can end up at the error message "Error: Login succeeded, but I don't remember why you were logging in [...]"
This happens when the user has earlier logged into a site on its https url, but then later navigates to the http url, and is prompted to log in there.
Workaround: Set redirect_to_https
.