Spotted prints - HP Designjet Z9

Hi, everyone

I’ve just bought a brand new HP Designjet Z9. I'm a macOS Big Sur (11.0) user and I have the raster driver installed fot the printer. I'm having the following issue;

All of the prints made from fine art papers, Hahnemuhle Matte Fibre and Hahnemuhle Photo RAG 308g are coming out with spots that seems to be directly from the printer. We have made several tests and we got the same results. We have changed the starwheels settings to “up”, which could fix the issue, but no. We have created a custom paper type, with different settings, also with no success. I will post an image of the spotted prints.

Does anyone have an idea of what it might be causing this issue?

Thank you,