This one was a head scratcher for a few minutes, and I admit I had a little internal panic thinking something had broken my Pod.
When accessing the admin page after putting in a load balancer, I was presented with a blue background and a blank white box. No logon options, no dashboard, no hope!
I tried all the Connection Brokers to no avail until I tried the https://localhost/admin option locally on the Connection Broker.
After a little digging it appears there is a new security option in Horizon 7 that is now enabled by default – RFC 6454 Origin Checking.