HA Proxy HTTPS offloading not working

[Stan]

I have a Unifi Controller running in an LXC container on a Proxmox server. It uses https at port 8443. I get the security error on my Chrome browswer and would like to use HA Proxy to tie a LetsEncrypt certificate to the Unifi Controller. I've watched a couple of YouTube videos, but can't make it work. I did successfully obtain a couple of LetsEncrypt certificates using the ACME service, one expressly for the Unifi Controller:
The certificate manager shows both. Here's the one for Unifi:

I've set up the backend as follows:
Here are three Frontend shots:



Can someone tell me what I'm doing wrong?