After a recent update to Transmission BT I was unable to connect to the WebGUI using the FQDN. After a bit of Googling I found someone else with the same issue. My fix was pretty simple on CentOS, I just had to modify /var/lib/transmission/.config/transmission-daemon/settings.json
Towards the bottom you'll see:
I just had to add my FQDN to the rpc-host-whitelist as follows:
Make sure you stop transmission-daemon before edit settings.json and restart it when your done.