It sounds like the thyristor, that is suppose to interupt the HV dc when sufficient light has been sensed from the on flash head sensor is not doing that.

In my metz CT-1 the lead from the pack to the head has three leads. One is negative DC. I thought that one is low voltage DC to run the sensor and dial calcuator electrronic in the head. The third is the HV dc plus.

I may be wrong in my past assumptions. Maybe the thyristor is in the pack, and the thirs line is the shut off signal line. That would suggest that the head gets power by bleeding HV off and stepping it down to run the head electronics.

I would inspect the sockets very carefully. On mine on the head socket there was arcing, and the socket had to be replaced. My CT1 pack came with the capability to drive two heads, so I scavenged the second socket from the pack and moved it to replace the socket in the head.