After PA is updated to MU #5, it is not possible to log into the branding panel using an additional admin account. After clicking the Websites tab of CCP, the login page is shown and it is not possible to log in as this user until user sessions are killed.
An admin's staff members cannot log into the PA Control Panel more than once at the same time. This leads to the above issue, caused by software issues #PPA-1171 and PPA-2265.
The fix is included MU #13.
Update PA to the latest version:
# /usr/local/ppa/bin/ppa_update --install-updates