You're not the only one to have this problem, but I haven't been able to duplicate it. I haven't really had a chance to look through the code yet either, though. I will try and have a look as soon as I can.
One thing you could try, around line 14461447 of class.tickets.php, change this:
if($var && !in_array(strtolower($var),array('email','phone','other')))
$errors='Invalid source - '.Format:($var);
to this:
if($var && !in_array($var,array('Email','Phone','Other')))
$errors='Invalid source - '.Format:($var);
Off the top of my head, I can't see what else could be causing it. That source list isn't dynamically generated.
This error just doesn't make sense.