This may relate to interesting feature on /etc/hosts file.
If there is a "manually" inserted host called "server.com" before the automatic line like this:
10.11.12.13 server.com
# --- BEGIN PVE ---
10.11.12.14 s1.server.com
... then Proxmox tries to add also s1 before the first "server.com" line.
After reboot this happens again... and again...
This will eventually result in something like this...
10.11.12.13 s1.server.com
# --- BEGIN PVE ---
10.11.12.14 s1.s1.server.com s1
*** smile ***
