Registration Help!

I have an event registration page set up, and so far we've had 50+ people register successfully. A colleague reached out saying that he was unable to register and kept getting an error. He also stated that this has happened for other trainings were he's tried to register with us. I looked at our page and everything seems fine. There are a few of his co-workers that have successfully signed up for the event with same (@___.org) email, it's just his doesn't work. I also tried to register for him myself with his information and got the error: "We're sorry but we've experienced an error processing your request. Please contact the Event Organizer." Any thoughts?