We checked the strange behavior of the HV dividers again. The Ohmlabs and the PTB divider are in good agreement at 120 kV and 150 kV. There, both dividers deviated by 0.35% from the value on the HKR multimeter. Ohmlabs divider yields higher voltage.

At higher voltages (180 kV, 195 kV and 205 kV) the PTB divider showed jumps. The deviation between Ohmlabs divider and set value increased at higher voltages up to 2 %.  Ohmlabs divider was fluctuating at 10 V scale but center was stable.

We removed the PTB divider.

Afterwards, the fluctuation of the Ohmlabs divider are at the 1 V level. Deviation between Ohmlabs divider and set value is now 0.35 % at 205 kV. Same as in the low voltage range with the well-working PTB divider.