Help! Why does my Currenza C2 coin changer only accept the first coin?

I have a vending machine with an NRI Currenza C2 coin changer BLUE. When | insert €10 and purchase a product for €9 (e.g., by selecting "55"), I correctly receive €1 in change.

The issue: If I want to buy something else and insert more coins, the machine only accepts the first coin as credit. All subsequent coins immediately drop into the return slot, no matter their value. No matter which coin I start with, only the first one is credited, and the rest are rejected. Does anyone know what might cause this or how to properly configure or fix this issue?