As instructed here, I have added this line to
-A INPUT -p tcp -m tcp --dport 20000 -j ACCEPT
Upon logging in to Usermin, the page does not finish loading after several minutes of waiting. With the Authentic theme, the index page is truncated and has only 26 lines (it should have 90).
With the Virtualmin Framed theme, the entire index page loads (it is much smaller than Authentic's), but
/left.cgi?mail=1 does not finish loading. It is truncated to 22 lines (it should have 68).
I have also tried unblocking udp:
-A INPUT -p udp -m udp --dport 20000 -j ACCEPT
but it has no effect. This problem goes away if iptables is stopped, and comes back when iptables is started again. I have no trouble accessing Virtualmin/Webmin.
I am using Usermin 1.690, Webmin 1.801, and Virtualmin 5.03. This problem happens in all browsers tested (Firefox 47.0.1, Google Chrome 51.0.2704.103 m, and Internet Explorer 11.0.9600.18314).