Is it permissible to perform qunūt in the second rakʿah of the ʿIshāʾ prayer, and is the prayer of the congregants valid in this case?

The Qunut of the Imam in other than the last Rak'ah, or in the Isha prayer when there is no calamity, invalidates the prayer because it is an intentional act that is not part of it. The prayer of the followers is also invalidated if they knowingly follow him. As for the ignorant, their prayer is valid. And whoever does not follow the Imam in his Qunut, their prayer is valid.