Now I want to mirror the part without associativity. The second part has to be independent, because I want to cut a logo into the first part. When I just mirror it, then the logo is mirror-inverted and you can't read it anymore.
The problem is, that the "independent copy" (or something like that)- checkbox is grey. I cant uncheck it and make the second part independent...
When I need something of this sort, I do the following:
(1) Define a family table, with the "left" and "right" instances of the part.
(2) Do the mirror.
(3) Add a protrusion that removes the the geometry that was mirrored.
(4) Add columns in the family table to include or not include the mirror and geometry removal
(5) In your case, also add a column that includes or doesn't include the logo feature.
The alternative is to just copy the original part to something with a different name, then do the mirror and removal features in there. I never do this because it means I have to change two parts the same way when I'm iterating on design.
Mirroring in Creo has and likely will remain a not so great experience, particularly if you want to mirror a few features and not the entire model. The only operation I find more troubling is "scale model" where once you make the change (i.e. scale to 1.124 times current size) you have nothing, no feature, no record of what the scaling was.