Siglent SVA1032X VNA mode: BW and sweep time are grayed

[RoV]

Hi,
I am doing some measurements with an upgraded SSA3012X Plus in VNA mode.
When trying to measure the impedance of a 14 MHz XTAL on port 1 around resonance, I have found a very anomalous response  (see figure), although the instrument was correctly calibrated. The real series resonance is at a much lower resistance (a few ohms) and there is a parallel resonance around 15 kHz above the other: even a NanoVNA can show it.
My hypothesis is that the sweep time is too fast, but I can't change it, because the relevant setting is always grayed. I have tried with the BW setting (a lower value would force a slower sweep), but it is also grayed and fixed at 10 kHz!
I have tried other measurement setups (e.g. full-span, preset, S21 instead of S11, ...), but in VNA mode BW and sweep time are always grayed, both from the VNA stimulus page and pressing the BW/sweep buttons.

The manual refers to those commands as normally usable, only saying that the default BW is 10 kHz.