After rebooting, you may find you cannot ping or access another of your Zerotier hosts on your VPN eg by SSH.
The solution to this is as follows:
On both machines that cant access or ping each other, run the following to find their Zerotier ID
sudo zerotier-cli info
200 info **999xxxxx99** 1.6.2 ONLINE
Then, on each machine stop the zerotier service:
sudo systemctl stop zerotier-one
Next, delete the peer data for each machine on both machines:
eg in this case:
sudo rm /var/lib/zerotier-one/peers.d/999xxxxx99.peer
Then, having done that – on both machines that have the problem, restart the zerotier service:
sudo systemctl start zerotier-one
both machines should now be reachable in both directions via your Zerotier VPN.