CONTENTS_SOLID is, in my experience, a brush based error. It's caused if you make some kind of invalid brush (again, from experience. I could be entirely wrong, so someone correct me if so :P). I recently got one with a perfectly valid piece of geometry: it was textured entirely in nodraw, except for one face which was skip (I was testing something, don't laugh at me :< ). That created an invalid brush, despite the physical shape of the brush being entirely fine.
Just keep an eye on the brushwork.