Hello roberino,

Welcome to the SAS | EuroBonus forums!

This seems like an odd issue as I haven't had problems entering frequent flyer numbers on the SAS website. So I thought I'd give it a quick try on one of my bookings and low and behold, re-adding my already added frequent flyer number returns an error, saying it has an invalid format.

it seems that somebody tampered with the logic that checks the details of the number and determines if it is valid or not... As a workaround I would just call or chat with them and ask the agent to add the number to your booking manually as that functionality of the manage my booking appears to be broken.