I ran into some trouble installing Proxmox 5.0b2 on a HP DL360 G5. Apparently the ACPI implementation in their BIOS is broken, even in the latest BIOS version for this server. The problems manifested as a completely blank/black screen in the graphical installer and once I got it installed, errors booting Proxmox like the following:
I was able to resolve both issues by adding "acpi=off" as a kernel parameter from within grub. After you get Proxmox installed and booted for the first time, you'll want to modify /boot/grub/grub.cfg to automatically pass the "acpi=off" parameter. Every time you update the kernel you'll also have to modify the new entry in /boot/grub/grub.cfg. If you're unfamiliar, go to askubuntu.com and search for question number 160036 (I can't post links yet).
[Firmware Bug]: the BIOS has corrupted hw-PMU resources NMI watchdog: BUG: soft lockup - CPU#0 stuck for 23s!