It's possible that the spacers used previously as you described, scored the metal in such a way as to create a stress riser, which started the crack. I doubt that the chainring is about to "fold in on itself" (unless your real name is Magnus Backstedt)--more likely you would notice chainring wobble when this attachment lets go, although you might not notice it right away on a single speed as you don't have the front derailleur to "sound the alarm" with chain rub. If this tab has cracked, the others could also be compromised (by scoring) but not showing cracks yet, so the best bet is to replace it.