Jura D4 - acid coffee

Hello, we bought a second-hand Jura D4. After cleaning, descaling and changing the filter, we set about tasting it. To cut a long story short, we've tried just about every type of coffee, and every time, the beverage is acidic. We also tried changing the water to one with a higher ph, setting the grinder to the finest setting, but to no avail. It's to the point where you'd think lemon juice had been added!
The only coffee we found to our liking was the Kimbo Aroma Gold, with the grinder set to point 3-4.
Perhaps this is one of the only coffees that matches our machine and that's how it works? Still, it seems strange that the grinder setting has so little effect...
Or maybe it's a known defect of the D4?

Your insights are appreciated.
Thanks,

Michaël