Is it correct that this question is more closely related to connection compensation (in this case a load) for the model 590?
There are basically three components to this process: open, short, and load connection compensation measurements.
It seems you can't proceed with the load (known capacitance) compensation because the reading is invalid.
How did the open and short measurements go? What are the connections like?
Is your 590 is in calibration? If not, I would recommend calibration for the unit. This may resolve some of the issues you are facing.