Okay, so maybe that's not the problem.
It does seem very strange that I can boot the Debian 10 ISO, and a CentOS iso, but not an Ubuntu Server ISO...

I do have some other hardware that I could try later. I did notice something that I missed earlier. When I boot into the ubuntu ISO, I see a couple outputs in the console before it gets to the setup area.

[0.887324] vbd vbd-5696: 19 xenbus_dev_probe on device/vbd/5696
[1.172167] piix4_smbus 0000:00:01.3 SMBus Host Controller not enabled!

Maybe this is telling for what could be going on here?

@olivierlambert

I am running on a SuperMicro PIO-618U-T4T+-ST031. With a Supermicro X10DRU-i+ motherboard and dual Intel(R) Xeon(R) CPU E5-2640 v3 @ 2.60GHz.

Hello, I reported a bug and it was closed as not reproducible. I'm curious to know if anyone else has experienced this issue?

I have a fresh install of XCP-NG 8.1. I made sure the pool is fully patched. I have tested this with two known good images of Ubuntu Server 18.04. (18.04.2 and 18.04.4)
When I attempt to start this VM to install Ubuntu Server, I get to the language selection before the VM locks up. On the same host, I was able to boot from and install Debian 10.0 without issue. However, when I go back to any Ubuntu Server image, the vm locks up and I see 100% CPU usage on the guest. I would be happy to provide more information or test with different iso images.
Context

**XCP-ng: 8.1.0
Good behavior with Debian 10.0
Bad behavior with Ubuntu Server 18.04

Expected behavior

When I boot into Ubuntu Server 18.04 Live iso, I should be able to install the OS.
Current behavior

The VM hangs and gives the errors seen in the image below. "watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [lsusb:1887]" and "watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [sfdisk:1777]"