Open Hammer and go to "Map>Go to brush number", And then write entity#0 brush#21
Because "CreateBrush: 0...1...2...Entity 0, Brush 21: A 'Unknown Class' @(0,0,0)", That's the error.
So just look if you did something strange with that and the other brushes in the error.
[EDIT]
Oh wait! You're using q-tools. They are the original tools and they SUCK! Get zhlt instead. Because I think that the site you were looking at, only supports zhlt.
http://www.zhlt.info/