Just to confirm what seems pretty clear and obvious, but it isn’t: The original zynthian socket is really 5.5/2.5mm - ONLY!
These sockets are often made mutually compatible both with 5.5/2.5 and 5.5/2.1mm sizes. But usually the vendors/sellers don’t give you precise information. For the sockets it means to be physically 5.5/2.1, but tight enough to accommodate a 5.5/2.5 plug with the slightly larger central hole just as firmly as a 5.5/2.1-only plug. For the plugs it means they are technically 5.5/2.5, but tight enough to hold firmly also in a 5.5/2.1 socket with the slightly thinner central pin.
However, the Zynthian mainboard uses a socket which is physically 5.5/2.5. It means that it WON’T accept a physical 5.5/2.1-only plug with the smaller hole, because of its larger central pin (as happened to me) - in that case one needs a small plug convertor.