The Firewall page in OpenAdmin loads an iframe that pulls data from a Perl file used by Sentinel Firewall (formerly CSF). If this Perl file does not have execute permissions, OpenAdmin cannot run it when the page loads, resulting in an empty iframe:
This problem can happen when updating only OpenAdmin manually using: opencli update --admin.
Solution: Restarting OpenAdmin resets the file permissions, fixing the issue:
service admin restart
After the restart, the Firewall page should display correctly.