Login screen is shown when try to manage any subscription through PPA or open Websites
Issue is caused by incorrect configuration of IP access restriction management, it was configured as per instructions from article but server IP address was not added to the list with "allow" option. The behavior described in the article is not supported by PPA as there is not ability to restrict access to PPA Control Panel.
Remove the record from 'psa.misc' table:
# mysql -uadmin -p`cat /etc/psa/.psa.shadow` -Dpsa -Ne"delete from misc where param='access_policy';