Created attachment 215089 [details]
Created attachment 215090 [details]
Created attachment 215091 [details]
Created attachment 215092 [details]
Recompile VirtualBox. Its kernel module does snot match your new kernel.
I have installed VirtualBox with
# pkg install virtualbox-ose
on top of a fresh FreeBSD 12.1
The same setup used to work in 11.3
Right. This is not guaranteed to work, unfortunately. To be safe, you always need to compile kernel modules yourself. Modules from package may or may not work. You can read a lot about that on the mailing lists and forums.
Created attachment 215120 [details]
Replacing vbox kernel modules with self-compiled has not fixed the issue.
When the host (FreeBSD) crashed, the guest (Ubuntu 20) was reporting numerous I/O errors. Unfortunately, guest has not saved any crash logs.
Also I noticed that NAT networking does not work with both stock and self-compiled modules. Perhaps this is caused by
module_register_init: MOD_LOAD (ng_vboxnetflt, 0xffffffff82ab0c50, 0xffffffff82aa7960) error 22
/etc/rc: WARNING: Can't load vboxnetflt module.
Not sure if it is related to the crash.
What source tree did you use when you compiled the modules?
Is it an exact match for the kernel that you have installed?
What would be a proof for that?