Haven’t looked into Ubuntu for years, because…
(https://www.whonix.org/wiki/Host_Operating_System_Selection#Recommended_GNU.2FLinux_Distribution)
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:
https://www.whonix.org/wiki/Troubleshooting#System_Logs
Maybe you are “lucky” and could watch systemd journal log when this issue is happening?
https://www.whonix.org/wiki/Troubleshooting#Watch_Systemd_Journal_Log_of_Current_Boot
Otherwise if above does not help, you could Enable Persistent Systemd Journal Log:
https://www.whonix.org/wiki/Troubleshooting#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:
https://www.whonix.org/wiki/Troubleshooting#Check_Systemd_Journal_Log_of_Previous_Boot
Please also read https://www.whonix.org/wiki/Free_Support_Principle.