I recently upgraded to Proxmox Virtual Environment 9 and was dismayed to see the server just hung forever after saying disks were initialized. A boot in debug mode (advanced grub boot option) revealed that systemd was hanging forever on waiting for the network service to start.

My first trick was to give the networking service a timeout by modifying /etc/systemd/system/network-online.target.wants/networking.service and adding TimeoutStartSec=90sec to the [Service] section. This at least allowed the system to boot normally and not hang anymore.

After boot, however, networking did not work. After much troubleshooting I found this article explaining it has to do with a post-up command I had for my mesh networking interface: post-up /usr/bin/systemctl restart frr.service

Once I removed that line, I was able to boot normally.