This is a bug caused by a recent change in the source code. You can revert to a slightly earlier version of the code with something like this to avoid the error --
git checkout HEAD~2 packages/xen-api/src/index.js
yarn; yarn build
sudo systemctl restart xo-server.service
Adjust as needed depending on the method you used to build XO from source.
OK, worked it out. I had a private lan with a vlan address of 0 on it.
I removed all of the machines and removed the network, and have now managed to attache the server as a second host to the pool.
Let the fun begin.