Can't send money via Apple Cash, says "The recipient is no longer valid."
I'm trying to send money to my son. When I click "send" in the Apple Cash app and it takes me to Messages to send, I get this error: "Cannot Send this Payment. The recipient is no longer valid." He has received money from me this way recently, and just a few days ago he sent me money via Apple Cash. My balance in Apple Cash more than covers the amount I'm sending. We went to the Apple Support page "If you can't send or receive money with Apple Cash," and everything checks out (we've made sure everything it suggests is correct on both of our iPhones; he should be able to receive money from me per that page.) We've restarted our phones, just in case. I'm still getting the error. Any ideas about what's wrong? Thank you.