I had this same problem this week.
I don’t think blender 2.8 will knife project if you are using a 3D object as the cutter.
In blender 2.8 when you select both objects @theluthier they both go into edit mode (unlike in previous versions of blender) and I also got the message it would not work and there was no cutting plane selected.
Is there a new workflow in blender 2.8 with this tool. I could not really figure it out.
I ended up just doing a boolean intersection lol because I couldn’t get it to work.
