This is an old thread, but I ran into this myself recently. While there is a link to deleting the entire XO configuration, I think I fixed it with a less drastic solution. Remember: I'm just a random dude on the internet posting dangerous commands to try. It worked for me, but your mileage might vary.
I run Xen Orchestra in a container (Xen Orchestra, commit e8733 at the time of writing). So I got a command line in the container with:
docker exec -it xoa bash
Then I ran redis-cli to get a redis command prompt. I typed KEYS * to get a list of keys. One key I saw was: 7) xo:server_host:172.30.0.214". That's the IP of the host I was trying to join (the master of a single-host pool). So I ran:
del xo:server_host:172.30.0.214
Then I restarted my container with docker restart xoa. After that, I was able to successfully add the host to Xen Orchestra.
Maybe this will help someone else. It got me working again.
