Hi Romano,
FYI I found the cause of the error and how to remove it. In case this is of interest to your IT team
It is related to the field “country of residence” in passenger details screen (see attached).
Even though this value looks already set ok when you first go into the screen, you need to re-set it again.
You need to select some other country first, then change it back to correct country, then you can proceed with the purchase without the error.