To me if feels like this error is caused by the fact that the connection in IST was already something like 60 mins. and that the changed schedule would have cut that to the point where the system was not allowing such a short connection time. Of course it would help if the system would just say that as opposed to just generating some error message that leads to nowhere.

Anyway, schedule change means a free change of flights so I just moved my mom to a flight the previous evening which is better anyway. Win-win in the end for everyone and problem solved.