What is the ruling on praying behind an Imam who sometimes makes mistakes in reciting the Qur’an, such as pronouncing the letter “Kaf” with a full mouth (tafkheem) in “At-Takathur” or saying “fa-haddatha” instead of “fa-haddith”?

If the Imam does not mispronounce Al-Fatiha in a way that alters the meaning, and he does not intentionally mispronounce anything outside of Al-Fatiha in a way that alters the meaning, then his prayer and praying behind him are valid. The aforementioned mispronunciation does not invalidate the prayer.