Since 1.8 version server can no longer "force" player to open CommandBlock in specific location by using "open tile" packet. There is probably some kind of "instanceof" check in client that not allows it anymore(opens sign GUI instead of CommandBlock). I was using command block as GUI element that allows me to get any kinds of inputs without using chat. I know that you have some kind of policy to ignore such bugs, cause it stands on modified server, but only you can fix it, so please don't just change it to "won't fix" without even checking. It shouldn't be so hard to fix it, only couple lines of editing to make it back as it was. Thanks.