Happened to me today as well. It seems to be an error that Xcode missed before compiling, resulting in a bad function in the executable (I may or may not have used the correct terminology, I'm fairly new to app development, sorry) - I've rewritten one of my functions and suddenly it started to work again. Until it fixed, try to delete / comment out chunks of your code (one at a time, of course) until you find where the problem is, then try to fix or rewrite it. Managed to fix that twice this way.