Online payments suddenly stopped working

We’ve been successfully using online payments, the last one processed on 12/26/18. But when users try to RSVP for an upcoming event ($10 + $0.61 fees) they can enter their credit card information but when submitting the payment they receive an error stating “This account cannot currently make live charges. If you are a customer trying to make a purchase, please contact the owner of this site. Your payment has not been processed.”

Well, I am the site owner and do not know the cause. The online payment settings are unchanged. It happens for multiple events, for multiple people. I created a new event and it fails for that too.

Anyone know the cause of this message?