I now see that you have Europe Central map on board. You've perhaps not understood what catymag is suggesting. As other possibilities have been eliminated and the card works in another unit, it may be that the gma & unl codes for Eu C map in the internal memory of your zumo are interfering with the proper operation of the unlocked all EU map on the card.

Like any other card-capable Garmin unit universally, your zumo can read a detail map on the card in Garmin root folder named gmapsupp.img. You have the naming and path correct as you see it in map info to select it, so then please try what she's sensibly suggested, back up the EC .img and any .gma/.unl files and remove them entirely from the unit as de-selecting the CE map may not be sufficient.