After some testing, the Trace is back on for some reason. Changes: I cleaned the dirt around in the HV area. Also lifted one end of R4124 to measure it and it was fine. Re-soldered with sliver bearing electrical solder. Then I noticed a loose wire in the HV area near the PCB edge where C4016, C4020 and C4025 are connected to ground. One end of the wire is soldered to the common ground of those capacitors. The other end of the wire was connected to the neighboring PCB, the Vertical Pre-amplifier board. I re-soldered the broken connection which is also a ground on the Pre-amplfier board. Since both boards are already grounded, why this extra connection? Possibly ground out stray signals of some sort or maybe prevent a ground loop? And why is the trace now back on?
Just in case i affected some other intermittent connection, I poked around the HV area with a nylon RF tuning screwdriver, and didn't find anything untoward.
Ah well, at least the trace is back on.