I am having the same issue. We would like to test Proxmox. Some people here want us to use ovirt but Proxmox was the hypervisor of choice in my last position.
I have installed Proxmox on two separate R610 servers, everything installs well but I cannot access the GUI from any browser on two separate desktops.
Wireshark shows that there is a response from https://192.168.11.16:8006 but the browser messages below show "establishing secure connection" and then above in the webpage "This site can't be reached. This site took too long to respond"
I resolved the issue. The issue was that I was installing from USB and I had used either Rufus or Unetbootin to make a bootable USB from the Proxmox ISO. The USB turned out to have a couple of pseudo drives and while proxmox appeared to install correctly, not everything installed. I made a new USB using ImageUSB (https://www.majorgeeks.com/files/details/imageusb.html) and everything installed correctly. Thanks for your help Wolfgang.