Trying to send a group message on iPhone results in “promo code not valid” error

Today on a group ride I opened the “send group text” box, entered a message and pressed send, and the box title changed to “enter promo code” and I got an error "promo code not valid, press [ESC] to cancel.” There is no escape button because I was using zwift on an iPhone and my avatar coasted to a stop even though I was still pedaling. Thus I lost the group while I tried to figure out how to get the message off the screen. The issue recurred several times but I seemed to be able to keep my avatar pedaling the other times (I think? I was busy trying to get the message to go away). I was able to confirm the title of the box indeed changes after pressing send and found that sending messages did work intermittently.

I’m using Zwift 1.32.1 with an Elite Tuo Smart Trainer.