Hi Michael,
thank you for your answer. Unfortunately this won’t solve the problem. The material is checked in the viewport and I have to say it again: It shows the gradient in general but only in black and white – not the colors (!) of the gradient. This is really weird.
Only when I render I ALSO see the colors not just the black and white version of the gradient.
It will show me solid colors any time. I really don’t know what’s the issue.