Riese & Müller Packster70 - vibrations

Hi everyone,
Hope nobody minds if I write in English as I don't speak German.
I have a Riese & Müller Packster 70 since December 2022 (4,700km in total so far) and recently, I've been experiencing very strong vibrations when I'm going fast (>45km/h).
It's systematic, i.e. every time I go at this pace, the cargobike starts vibrating and the vibrations are quite impressive & scary (I feel them in the handlebars). Children in the cargo also feel them.
It never vibrates when I'm going slower than 45km/h.
I was wondering if anyone had any idea what could be the reason. The bike is recent & well cared of, it's overall in excellent shape.
I live quite far from big cities and only have one bike specialist, who has no clue about the reason behind this issue.
Many thanks for your help.
if the bike is well maintained I suppose there is no slack in the headsets, steering and rear suspension joints if applicable? On a cargobike if this happens I would mostly look at the steering. We've had shimmy on delivery bikes with both too slack and too tight steering cables
The older models of the packster 70 had been recalled 2021 due to similar problems with natural frequency.

The packster uses a cane creek viscoset to reduce the vibrations.
It's possible to adjust the viscoset for more or less damping.
both responses are extremely helpful. The cargobike is going to the bike specialist tomorrow, I hope they can fix the issue, but with your help it will be easier for them I'm sure :) thanks again.
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