Who shouted at Paul, "You are insane, Paul! Your great learning is driving you to madness!"?

Answer: Festus

During Paul's defense, Festus cried out that Paul was insane; Paul replied he was speaking words of truth and sobriety.

“At this stage of Paul’s defense, Festus exclaimed in a loud voice, “You are insane, Paul! Your great learning is driving you to madness!” But Paul answered, “I am not insane, most excellent Festus; I am speaking words of truth and sobriety.”— Acts 26:24-25 (BSB)