When I added site 13, I got the response "Site added - No sites found". The message portion of the Sites page indicated "Site added", and the site list portion of the page showed "No sites found" where my twelve sites had been listed moments before.
From my testing to date, this only occurs when I have my complete profile set up in the template used for the added site. I have tested individual elements, and not one of them is sufficient to cause the failure. I'm not aware of any resource issue which could cause this - I got the same behavior on a smaller, slower server with much less memory (I've got my PHP memory limit set to 2700M).
I've got tech support at my hosting provider (HostGator) working on this but I would desperately like another set of eyes looking at this problem.