I understand. Thank for all your time. I actually found something.
As I wrote in my first post today, the NIC name changed when I upgraded. That also happened to the VMs, I just discovered. So I need to set th correct interface name in the netplan config file. Tried it on my first VM, and that seems to do the trick. Got the correct address from DHCP, and it is online!
Now I´ll just work through the rest of them and see if that solves everything.
Again, thanks!