In case this save someone hours of frustration. I recentlty tried to dust off an old kubernetes POC running on virtualbox VM's. I could not get anything to work right until I realized that Virtualbox somewhere in v6.x started to ONLY support 192.168.56.0/21 for their host-only networks. Even though my old vboxnet's where still there and even configurable!
My kubernetes POC had primary NAT and secondary host-only networks. I still had to re-initialize my cluster an lost all my POC work even after I fixed the networking but at least this may point you in the right direction. To allow my 172.20.0.0/16 network I added this to the config file:
# cat /etc/vbox/networks.conf
* 172.20.0.0/16 192.168.0.0/16