I've had this on a few plants, and I have assumed it is rust (a form of fungus). I have been treating the plants in question with systemic fungicides that are listed as good for rust. I think they are Bayleton and propiconazole, both group 1. Here is an interesting site:
http://www.chattanoogarose.org/MOA%20Entire%20Paper.htm
I have also cut off the leaves with the rust, as it is not possible to tell if the problem is slowly spreading with the spots still on the plant. With Paphs, I under stand that rust can do a pretty good job of killing the plants, so I don't want this in my collection at all.