Issue with HAproxy and Proxmox server after upgrading from 21.1. series to 21.7

[chriskirsche]

Hello together,
after upgrading from the 21.1 to the 21.7 series I've run into an issue with HAproxy and an only local available Proxmox Server. After the upgrade the Hostname is still perfectly resolved and the website starts to load. But some of the required *.js files don't reach the calling browser.
If I call the website by its IP-address, everything works fine (besides the SSL-check of course).
 
HAproxy is setup together with Unbound so that I can use one SSL-certificate for all the web services in my network.

And it works for the following services:
External reachable:
Nextcloud: https://nextcloud.mydomain-external.com

Internal reachable:
Unifi Controller: https://unifi.mydomain-internal.com
Plex Server: https://plex.mydomain-internal.com
ecoDMS: https://ecodms.mydomain-internal.com
OpenHAB: https://openhab.mydomain-internal.com

Not working:
Proxmox Mangement website: https://proxmox.mydomain-internal.com

To check if the rules for the Proxmox are screwed up, I just changed in haproxy the IP and port of the unifi controller to the Proxmox IP and port and tried to open up Proxmox website by using the https://unifi.mydomain-internal.com URL.
But the problem stayed the same. So my assumption is that the rules are setup correct.

I also tested different machines with different browsers:
Windows 10 with Edge, Chrome, Brave and Firefox
MacOS with Brave and Safari
iOS with Safari and Chrome

Next thing I did was to check what the developer console of the browser tells me what the problem is. Some of the *.js files aren't delivered to the browser (see attached picture).
When I call the Proxmox IP directly everything gets delivered to the browser.

Any suggestion what I could do to pinpoint (and hopefully fix) the root cause of the problem is highly appreciated.

Thanks!

Chris