This one somehow seems related to the + sign in the URL (encoded as %2B). I managed to reprodce your problem once and ended up with a space in the middle of the URL (which is a typical case of missing escape for it).
But I've been completely unable to reproduce it since :/
So there is a bug lurking there somewhere, but I'm not sure what it is. If it happens again, please try to re-trace your steps even closer, and in particular pay attention to if at some step a blank space shows up in the URL bar!