This is a known p5 problem, in the material room, delete all the nodes connected to the background and you will be able to change the background again. It works the same for pz3s load the pz3 then delete the nodes and that is it. When you download the latest patch for P5 (if it's still available), there should be a README file. All you have to do is read it. And make a couple of clicks in the right places... takes just a second to fix. (this info was gathered from earlier threads about this problem) -jjsemp