Proxmox web interface became unavailable and was not listening on port 8006. Also unable to backup a running KVM. This all pointed towards a missing/corrupt package.
To fix, the following was carried out:
1. Update repositories
Insert the relevant sources from here: http://pve.proxmox.com/wiki/Package_repositories#Proxmox_VE_No-Subscription_Repository – In this case it was the no subscription for Proxmox VE 4.x repositories.
Now run the following commands:
apt-get update apt-get install pve-manager
Pve-manager seemed to be corrupt. Updating to the latest version fixed the issue.