This component, Q233 on TiNs photo, became very hot when driving the output to a high voltage. As the voltage reading was always wrong, i eventually found out that op-amp U219 (OPA124) had a large voltage difference between the inverting and non-inverting input. As this op-amp is a simple high-impedance voltage follower, i tested it out of circuit to make sure the output was not externally clamped. With success, the output was stuck at -15V