@JMSolo, this bug is slightly different from the bug I was encountering on my own server. This bug is indeed because of the interface being set incorrectly.
In my specific instance, I had thrown the panel onto an existing installation of Debian 8. My issue didn't look similar to the screenshot (in fact I was able to replicate the screenshot by changing the interface to a bogus value). In my specific case, the bug was that you would click Additional Bandwidth Details and the dropdown would not drop.
I later realized that this was specifically in relation to the if $username == "$master" and somehow the comparison returns invalid / false.
Yet if($username == "$master") doesn't seem to compare ( http://puu.sh/pivmg/d5aff1e99f.png , http://puu.sh/pivzF/69632e89ac.png ) Given that I've only run into this issue on a server I back ported the panel to, I'm going to chalk it up to something I don't really care about and leave it alone.
http://puu.sh/pivF4/aa1b375c38.png -- This is the culprit here. I don't understand php well enough to understand why it isn't working here but just fine everywhere else.
Edit: Further investigation shows the $master variable is tacking on a space somewhere so $master = "liara " and $username = "liara" (spoiler alert: the extra space doesn't exist in the master.txt file ) . The solution here is to trim extra whitespace from the variable in inc/config.php
`$master=preg_replace('/\s+/', '', $master);
I do not think this needs to be implemented in the repos because this strange behavior has not cropped up on a single other installation.