The voltage of the DC/DC converter (-720V) of the QP1 driver unit went below the limit.
Check the QP1 driver -720V power voltage. The following causes are inferred.
(NOTE: To restart, turn OFF the plasma once.)
S24V is not supplied (Cover is open).
QP1-related cabling came off.
QP1 rod, or QP1-related wiring (including the vacuum chamber) causes a short in other metallic parts.
Either the QP1 Tank PCA or the QP1 controller PCA is faulty.