I think Elektron had problems with coated buttons turning sticky for a while, would fit the date it was produced. Electron (or Thomann) should send you free buttons in that case.
Elektron device: on several of the boxes I had the volume knob felt wobbly, but when I opened the unit I found no problems with the pot, and they always worked fine.
Edit: here’s a thread about sticky keys on the D1
Good that they already said it was not refurbished, because now they have no basis for the claim that it’s a unit that was “replaced under guarantee”. But if they still do, demand to know what was the cause for the replacement, how the unit was repaired and tested, which parts were replaced, and why nobody bothered to clean it and fix the pot.
Isn’t the serial number also in the invoice or shipping documents? That would settle that.