Comment 3 for bug 1398977

One more note on how to reproduce this is I was editing the hostname to customize by pressing backspace which brought it into an invalid hostname with a '-' at the end which could mean that is was an invalid hostname at one point and ubiquity was not seeing the new hostnames after that as valid.

I immediately tried to install again and was successful without editing the hostname and using what automatically fills in the box. I can still get the hostname I want postinstallation by entering sudo editor /etc/hosts and changing the 127.0.1.1 to your desired hostname. I did have to reboot as clicking back would still have an invalid hostname.