Hi,
Thanks for your quick replies! 🙏 I managed to find and solve the problem myself, it was an user-error again.. I had applied the label material (with the opacity mask) only to the "front side" of the label object (only to the "polygon normal side"). Then I used a second WHITE material on the backside of the label, but the white material didn't have the opacity mask loaded... so then I guess the 'white backside material' was still casting shadows/reflections on the object behind the label, even if it was not visible.
When I deleted the white material OR loaded the same opacity mask to the backside material, then the shadows disappeared. Same if I changed so that the label material was applied to "both sides" (see screenshot).
Thanks again, but my mistake!