Error encountered when inputting birthday: "Unable to verify your identity. Check your details and try again."

Went through the process of getting on an xfinity plan for internet. Selected the various add-ons, and got to the stage where I was being asked for personal information. I input my birthday, and the error message "Unable to verify your identity. Check your details and try again." appears. I try various different ways to write my birthday (MM/DD/YYYY, DD/MM/YYYY) and none of them work. Upon reload, the error message "We're sorry. Please contact us and one of our friendly and knowledgeable representatives can help" appears. I restarted the entire process of getting on a plan, and to no avail. Lastly, I try looking at my cart on xfinity.com. There is a red dot over it, which implies something is in there. However, when I click it, all I see is the same "We're sorry. Please contact us..." error message.