Take your compile log, put it in here:
http://www.interlopers.net/errors?page=errors .
Tell what it says, because this most definitely looks like a leak (or missing materials, which I doubt).
Wait, this is the stock TF2 skybox? You might have missing/corrupted game files. To fix this, close all applications using TF2 content (Hammer, CompilePal, GCFScape, etc), then go to Steam->right-click Team Fortress 2 in the game library on the left->Properties->Local Files->Verify integrity of game cache. This will replace any missing or corrupt files.