Update: I called again, i guess to the Elite Plus line this time (the last few times I was too lazy to input in my TK number).
From this call, one weird thing which I cannot understand - but I still accepted it was that Krakow (which is my inbound final destination) belongs to another region - and therefore it was because of all my previous attempts were deemed NOK. Strangely my original departure point was WAW, which was OK to be changed (now to AMS).

In the end I just accepted this. But I am still very curious what make KRK so special - according to the M&S region definition, there are no such exception. Curious if ISTFlyer with his wide knowledge knows....

Thanks again to all for the advices/help!
Cheers!