Is the prayer of the congregants valid and accepted behind an Imam who was a fornicator then repented?

You have done well to repent from fornication (zina), which is one of the major sins. We advise you to recite the Quran frequently, engage often in the remembrance of Allah (dhikr), and associate with righteous Muslim women. Your leading of women in prayer is valid if it occurred after your repentance. If it occurred before, the preponderant opinion is that prayer behind a fasiq (transgressor) is valid.