Is it permissible to pray behind an Imam who makes a mistake in reciting Al-Fatihah in a way that changes the meaning, and what should the worshiper do in this case? What is the ruling if the worshiper is unsure about the Imam's proficiency in reciting Al-Fatihah?

One who mispronounces a letter of Al-Fatiha, like a lisping person, cannot lead the prayer except for someone similar to them, according to the majority of scholars. However, the Malikis hold that it is valid to follow such a person if they are unable to learn. As for someone who makes a grammatical error (laḥn) in Al-Fatiha that alters the meaning, despite being able to learn, their prayer is invalid, and it is not permissible to follow them. The fundamental principle is the validity of the Imam's prayer, and one should not succumb to whispers (waswas). If you are certain that the Imam's grammatical error alters the meaning, then do not follow him; pray behind someone else, and advise the Imams who make such errors.