I had the same problem today on my Mac (2.4 GHz Intel MacBook running OS 10.5.6 running Express Invoice 2.04). I think it happened after I had to force quit the software because of a bug. When I tried to launch, it would just sit in the dock and nothing would happen. After restarting the computer, the app still would not launch. I had to re-install the software (luckily I didnt have any invoices yet so I didnt lose any data)