I get those bogus messages as well after updating to a recent AMD driver release (18.104.22.16882).
I think I ran into this same issue.
I have an FBO with one render buffer attached to GL_COLOR_ATTACHMENT0 and one render buffer attached to GL_DEPTH_ATTACHMENT. The format for the color render buffer is GL_RGBA8. I had to use GL_DEPTH24_STENCIL8, not GL_DEPTH_COMPONENT24, for the depth render buffer. This one change made it so that the blit no longer threw an Invalid Operation error. The weird thing is that my window's config attributes don't request a stencil buffer size.
I'm not sure if this is an ATI driver bug or what, maybe a moderator could shed some light on this.