Can no longer access CSF firewall.
CSF shows for a second or two and then the browser is redirected back to webmin home page.
This problem is also reported by others in the forum post: https://www.virtualmin.com/node/42062
Not sure if it is the latest csf causing the problem or the webmin theme. One person reported that "Using the latest Authentic Theme 18.10 Change theme fix the problem."