Haven’t looked into Ubuntu for years, because…
I guess nowadays Ubuntu is using systemd as init system and thereby journalctl for logs.
So this is more a question for your host operating system (Ubuntu) and/or VirtualBox. Nothing that Whonix does should be capable of crashing the host computer. Otherwise these by definition are Ubuntu and/or VirtualBox bugs.
Our instructions for viewing logs work on Debian (and Whonix, since it is a derivative of Debian). Dunno if they would work on Ubuntu too. Rather than doing them in a VM (which you could try too), you could try them on the host. See:
Maybe you are “lucky” and could watch systemd journal log when this issue is happening?
Otherwise if above does not help, you could Enable Persistent Systemd Journal Log:
(Dunno if Ubuntu by default uses persistent systemd journal log or not.)
And then after freeze and after reboot, Check Systemd Journal Log of Previous Boot:
Please also read https://www.whonix.org/wiki/Free_Support_Principle.