Didn't want to spam the bugtracker, because they might be all fixed.
Host OS was Ubuntu 18.04 LTS.
Virtualmin has been installed in Virtualbox and Hyper-V both, encountered bugs under both, so here it goes.
[ CRITICAL ] Installation is just simply broken with Hetzner Cloud + Floating IP. Installation of floating IP before even install is highly recommended. Dual IP is definitely NOT recommended.
[ CRITICAL ] Nameserver is borked.
sudo apt install resolvconf then
nano -w /etc/resolvconf/resolv.conf.d/tail, then
resolvconf -u and you have a working DNS resolv. HAPPENED ON HETZNER CLOUD VPS - did not happen under Hyper-V.
[ CRITICAL ] FirewallD does not add (or re-add) network interface(s) to zones, ie.: public. Resulting in ALL open ports. FirewallD should check if ALL interfaces have a zone ALWAYS. Using the CLI or GUI to add the zones properly works. HAPPENED ON HETZNER CLOUD VPS, IN HYPER-V GUEST.
[ CRITICAL ] OpenDKIM does not run properly until you issue the following commands. Yes, you enable it in the panel, you set it up properly, but it doesn't matter. The /etc/default/opendkim file will get ignored until these are issued:
> systemctl daemon-reload
> service opendkim restart
[ HIGH ] FirewallD doesn't work in the control panel. it has to be re-enabled as boot time process, THEN it runs/works. HAPPENED ON HETZNER CLOUD VPS.
[ HIGH ] Upon copying a Let's Encrypt cert to Dovecot/Postfix, they'll refuse to function further until you do a reboot. I am sure restarting services work just as well, but meh.
[ NORMAL ] Low CPU speed can cause the clamav step during install to timeout. Timeout/wait time definitely should be increased here.
if DNS is borked:
sudo apt install resolvconf
sudo nano -w /etc/resolvconf/resolv.conf.d/tail
sudo resolvconf -u