In principle, if the Imaam has beliefs of Shirk, for example, regarding Rasulullah [Sallallaahu Alayhi Wasallam] as Aalimul ghayb like Allah or regarding Rasulullah [Sallallaahu Alayhi Wasallam] as Omnipresent, then it is not permissible to perform Salaat behind such a person.
If the Imaam does not have beliefs of Kufr and Shirk, then Salaat behind him will be permissible.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai