Something on the JavaScript side perhaps, because when I click it, I don't see any request being sent to Foreman. Chrome stable.

Nitpick: The "Error generating IP: Not Found" error is being inserted in the field itself on error (e.g. proxy is not reachable). This is not only nice from the UX standpoint, but when you try to create new host with proxy unavaiable and you don't even open the interface form and submit, the IP is kept set to this string, which causes validation error. We should totally leave IP field empty on errors showing this message in a bubble or something.