That response has no information at all. The written notification will have a reason (though it's a pretty broad category).
Recon number is in the wiki for the other thread:
(800) 695-5171 – Personal Application Status and Reconsideration Line with live rep
(866) 606-2961 – Reconsideration Line.
Executive Office - useful for getting a more empowered agent for reconsideration purposes: CitiBank Executive Review Department, P.O. Box 6000, Sioux Falls, SD 57117
I'd probably call one of the numbers and see if you can figure out what's going on but I'm not sure if that's good advice.
The end of the FAQ in the other thread says
22. I was following the timing rules but got denied for Multiple Applications for Credit ("MAC"). Why??
Multiple Applications for Credit (or MAC) normally means that you violated one of Citi's application timing rules, like applying for more than two cards in 60 days. Because Citi has trouble counting, applications have been denied in the past if they were too close to 60/90 days. However, starting in August 2016, people have been somewhat randomly getting this denial when applying well outside of the 60/90-day range. CSRs frequently say that you can only apply for a single card in a 60-day period but this is not the case and people who have gotten this denial have gone on to successfully apply for 2 cards in 60 days or less.
You can try to request reconsideration or apply again as if the denial was an approval. You do not have to wait 60+ days from the denial to apply again.
I'm not sure if this is your reason or not; the person on the other end of the recon line may be able to tell you. Good luck!