The acidity of the cheese was too high. Given that the recipe has no starter culture, this means that you were either using raw milk, or milk that had gone off. In both cases, you need to use very fresh milk for halloumi because it needs to have a fairly high pH. In the case of raw milk, you might consider thermising it, just to stop the naturally present bacteria from acidifying the milk (just raise it up to 63 C/145 F for about a minute and cool it back down again). However, even using raw milk, it should not have acidified to the extent that you saw it unless the milk was on the brink of going off.