Answer
(Fatwa: 1582/1248/B=1431)
You should offer salah behind the imam who recites the holy Quran correctly, though he is sinner. However, your salah would be makrooh behind him. But the Imam who does not recite the holy Quran correctly, he may commit mistakes which may invalidate the salah. Thus, it is most probable that salah behind him is invalid. . It is said that when a man is caught between two problems he should choose the easy one.
إذا ابتليت ببليتين فاختر أهونهما.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband