My Proxmox 4.1 wouldn’t reboot. A quick look at the logs showed this was caused by the watchdog. My Supermicro (like HP) uses an IPMI hardware watchdog, but the Proxmox default is softdog.
In fact, by default, Proxmox disables loading of the ipmi_watchdog kernel module. And if you remove it from their blacklist, they just overwrite on the next update.
The fix is to uncomment the ipmi_watchdog line in /etc/default/pve-ha-manager, like so:
# select watchdog module (default is softdog)
After you reboot, you should see ipmi_watchdog output by lsmod.