The last 5 starters that I made have all had a blow out and I can't figure out why.
I am using Pilsen or Extra light DME @ 1 gram per 10ml of water (120g for a 1200 ml starter). I boil it for approximately 5 minutes and put it into a 2000ml flask with a stir plate and a foam stopper. In the morning there is yeast all over the out side of the flask.
I used 3 types of yeast (1056,1272 and WLP001)
I've had the same result when I just you aluminum foil too.