I've been doing some detective work :p
First of all here's your fixed project: test_summon_fixed.capx
gatewayexplosion is showing up as gateway& #x1f;explosion in the XML. #x1F is the ASCII code for the Unit Separator which is Ctrl+_
So, you want to name your sprite gateway_explosion but you accidently type [Ctrl Shift -] instead of [Shift -] and you get this dodgy character. Next time you open the file C2 can't read it and gives an error.