The film's world-building suffers from too much ambiguity in the characters' motives and history (presumably saving them for a possible sequel), and the intricacies of the world are glossed over, making it all feel lifeless and fabricated.
A bunch of bizarre-for-the-sake-of-being-imaginative stuff happens in The Last Witch Hunter, much of it making little sense, and some of it that probably made sense but I hadn't the energy to figure it out.
Diesel is definitely not the kind of actor who can effortlessly coast through a film by dint of his rakish wit and charm. He needs a vehicle (ahem) that plays to his strengths. The Last Witch Hunter is definitely not such a vehicle.