I've had a bad GFCI breaker before. The one I got was too sensative. Run the spa for more than 30 minutes and it would trip. After checking for problems and finding none, I replaced it and never had another problem.
I know that doesn't help you, but sometimes breakers go bad. If it's an off the rack breaker, you can probably take to a home center for an exchange. Your supply house might even help you out. Either place is going to send it back to Siemens.
