Proxmox web frontend did not startup.

Problem

The problem is always DNS.

Analyse

Look in the systemd journal.

cees@pve02:~$ journalctl -b 2
...
Sep 03 12:48:45 pve02.cvdg.eu pmxcfs[1070]: [main] crit: Unable to resolve node name 'pve02' to a non-loopback IP address - missing entry in '/etc/hosts' or DNS?
Sep 03 12:48:45 pve02.cvdg.eu pmxcfs[1070]: [main] crit: Unable to resolve node name 'pve02' to a non-loopback IP address - missing entry in '/etc/hosts' or DNS?
...

Solution

Use real ip address in /etc/hosts

I edited my ansible files to use the real IP addressed for pve2 and pve03.