I just left it as it was -- it's pre-filled with
auto
Are you 110% sure about this? Because I confirmed a bug in which changing the case of the word "auto" to "Auto" or "AUTO" would indeed result in the port number being set to 0.
If you can reproduce this I'd very much like to see it as leaving it set to "auto" works on all 3 of the servers I just tested.
Actually, try logging in as admin and clicking Settings, then the "Defaults" tab. What is the port range for new servers set to? And are there actually any ports free in that range?