debian VM can't connect to network: arp error?

I created a debian VM and vmctl starts it successfully (thanks to
earlier threads on that topic!). At first, the network worked fine.
Then I don't know what I did, but after restarting the VM,
the network would not work (can't ping anything from inside it,
nmap can't see the VM at all). Even recreating
the VM image and starting that--no network. I noticed an error
on the host, in /var/log/messages from around the time I started
the failing VM:

/bsd: arp: attempt to add entry for 100.64.1.3 on tap0 by [ipv6 address] on tap1

...repeated many times. But I don't know my way around arp or tap interfaces.

Rebooting fixed it. But for next time, is there a better way for me to
address this?

Thanks much,
Luke Call