Hello,
My scope's input impedance is : 1 MΩ ±1%, in parallel with 12 to 13 pF (Picoscope 3204B)

The x1 probe is a picotech TA375 with input resistance equals to that of the scope and input capacitance of 57 pF + Cscope = 69 to 70 pF
The 20:1 attenuator is a Hantek HT-201 with an input impedance of 1.053 MΩ. But I don't know how the voltage divider with this resistance value could achieve 1/20 voltage attenuation? Also the Hantek website doesn't specify the input capacitance for this attenuator!

Thank you

 

Normally x10, x100 scope probes have 9MΩ and 99 MΩ impedance (within the probe), which combined with the series 1MΩ input impedance of the scope, achieves the desired x10, x100 ratio. This also has the advantage of decreasing the loading on the circuit being measured, with a x100 probe offering an effective input impedance of 100MΩ, rather than 1MΩ.

So it is almost certain that your x20 probe includes an internal 19MΩ, with an effective loading impedance of 20MΩ.