I am using a 10 Gig Ohm voltage divider so 20KV reads as 20V. It is buffered through op amp as unity gain follower to meter. Right now I am researching amperage. I think, maybe, the rapid change in voltage creates too much current for op amp. i=c x dv/dt. cap= 1uF, time=<100ns, voltage change is 20v.
I love those cheap little digital voltmeters, but they have way too low impedance for Gig ohm divider... hence the op amp.
I can try a diode on the 20v input if you think this would help.