Did you try:
- Changing the fog shader to a known one that works.
- Deleting the brush and re-creating it.
- Moving the brush to a new location (sounds like you did at #19).
- Copy just the brush and the surrounding brushes in to a new map, box it off and compile it.
- Remove everything except what you are working on and with, then compile again.
- Check the compile log for errors and remove them if found.
- Open the map file in a text editor (like Notepad) and look for an error or issue in the raw map file.
Just a few things to try.