http Access Stops (FirewallD Bug Report Maybe)


I have a VPS with CentOS Linux 7.2.1511 & Virtualmin 5.04.gpl & Webmin 1.821.
For unclear reasons, the http access to websites stops suddenly while the https to websites access is OK.
I have set up FirewallD with http service so that when I click Apply Configuration problem goes away. Yet it happens again after a while, no idea why.

Any idea where to look at or is this some sort of bug?