A recent micro-update from Parallels seems to have broken the login page on some Parallels Plesk servers. In my case, it was a Plesk 9.5.4 server running on Linux.
If you look in the Plesk web server (“sw-cp-server”) error log at “/var/log/sw-cp-server/error_log” then you should see entries such as:
(mod_fastcgi.c.2582) FastCGI-stderr: PHP Fatal error: Call to undefined function get_failure_redirect_url() in /usr/local/psa/admin/auto_prepend/auth.php3 on line 175
The simple fix for me was to re-install the Plesk micro-updates using the autoinstaller command:
Once this has finished, your Parallels Plesk login page should be working as normal again!