It took me four hours to try and make smbc today. It was rainy and humid but initially it set up and was stiff peaks. I added my vanilla, mixed again and after about a minute or two it started to turn gummy. usually the more you mix the better but in this case it ruined it. So I put it in the fridge several times to try and save it. When that didn't work i made regular buttercream. Just butter, crisco, PS vanilla and milk. Again, came together nicely, let it whip up for a few extra minutes to double in volume and it too started to turn gummy. Not whipped or thick but more liquid and stretchy consistancy. It tasted fine but I wasn't happy with the texture and it made in difficult to frost the cake. What could have been the problem. It took me almost five hours just to make the frosting about an hour to frost it. so frustrated when stuff like this happens. I wish I was a pastry chef so I would know what to do.
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It sounds like your powdered sugar might have more corn starch than normal. Is your supply different in some way? Do you think adding a little bit more milk might help? It's really hard to say without seeing it and not having had the problem before.
As for the SMBC, I really have no clue. Did you use fresh egg whites or egg whites in a carton? Maybe there were extra ingredients in packaged stuff if that was used. Maybe more butter will fix hhis batch.
I wish I had more difinitive answers for you. It's a very interesting conundrum.