I have the same problem after updating Virtualbox. Reinstalling, updating, reconfiguring kernels, nothing worked. This is problem only with Whonix, Windows virtual machine works like before.
WHAT WORKED HOWEVER was manually making Whonix by making new Debian 64 machine and adding .vdi file as a hard drive. Then I would imitate original Whonix settings and so far it works, this is how I’m answering your question.
Is it problem with .vbox file? I don’t know, importing new Whonix machines didn’t work, just manual add.
THAT SAID PLEASE ONLY DO IT IF YOU KNOW WHAT YOU ARE DOING! Who knows if I configured it correctly, it works and Workstation is fully torrified but I would consider this method as quick fix, I hope some Whonix Developer will look into it.