When you install Plesk 9 with all the features it enables SSO by default.  When you login to the Plesk control panel and enable the firewall module you will then get locked out of Plesk when you disconnect.  You won’t be able to login and may just be presented with a “Continue” button which will then time out.

URL: http://kb.parallels.com/en/5481

You can set the firewall to allow access to another port (SSO port), or, you can disable SSO with:

/usr/local/psa/bin/sso --disable


or

%pleskroot%/admin/bin/sso –d