TOPIC: Error on checkout

We're getting an error when a registrant adds multiple items to their cart. Each event works fine singly, but when both are added, a repeatable error shows up onscreen, they don't get their confirmation e-mail although they are registered.

Anyone else experience this? Site's on J3.40 and DTR 3.09.

UPDATED: It appears in one event only (the other event processes fine), and we thought related to a field. It appears now, it's related to a custom display of message on the screen for Pay Later. It doesn't redirect on either a custom URL or the onscreen message, and gives an error.

Inexplicably, the guest fields it was erring on, I removed them, reset them and it works fine now!