This one stumped me for a day. On a site that has been using BPS for some time without problems, suddenly one day the WP login screen does nothing except refresh the form on each login attempt.
No messages, no prompts, no errors, nothing just constant refresh with no action. It was impossible to login and reach admin. At first I thought the site had been hammered by bot logins and my server admin had disabled the login function. Eventually by long process of elimination, I added my own IP to line-181 of wp-admin/.htaccess and finally the login form functioned properly.
This was a very time costly and annoying issue. Made worse by not knowing where to start looking for the problem, and not being able to access admin.
Is this intentional? and without any notification of what had happened?