From what I've been able to gather, one of the ads wasn't appropriate for the target audience (Advertising a mature rated comic) so it's been recalled. Replacements are being printed though so it's only a delay, really.
It's E106 Eta. E107 Theta was originally going to be the robot's name, but it turned out that he'd already mentioned such a thing in the Light Mobius future and he didn't want to allude to concepts from those future storylines. So it was changed.