What is the ruling on praying behind an imam who errs in pronouncing some verses of Surat Al-Fatihah, and must I repeat the prayers I performed behind him and inform the worshippers of that, knowing that he does not accept admonitions?

It is not permissible to follow in prayer someone who commits an error in Al-Fatihah that alters its meaning. Therefore, one must repeat the prayer if performed behind such a person and avoid praying with him until he corrects his mistake. If he corrects it, then it is permissible to follow him. However, if the mistake recurs from him, the prayer should be repeated. The Malikis hold that unintentional error does not invalidate the prayer, whether it is from the Imam or the worshiper. If the error is intentional, the prayer is void; but if it is unintentional, it is valid. As for pronouncing the "nun" in the word (Ar-Rahman) with a dammah, it is not permissible to do so intentionally, but the prayer is not invalidated by it.