I believe we have identified the issue. Somebody sent us the dump file (maybe you?) and it seems the hostname is not resolved on time and iOS kills the application because it gets impatient.
We are working on a solution, if you provide us with your udid on support@zoiper.com we can add you as beta tester as soon as we have a fix. (The review procedure for Apple is rather slow, so the official update might take several weeks before it becomes available).