Hey guys!
I just worked out how to fix the notorious buildgame error that occurs while loading sky. It has in fact nothing to do with the skyboxes
Open your "Buildfiles.ini" and look for a longer path than all other media uses. It is likely that one of the meshes has a texture path saved in its codestructure that leads to a drive or folder you do not have. Once you found this file, you will have to import it (fragmotion is a good tool) and either delete that path completely or remove solely the lines until the name of the texture. (To make myself clear: If the path is displayed as: "C:/thegamecreators/fpscreator/files/texturebank/yourtexture_d2.dds. Remove everything except "yourtexture_d2.dds)
You can also remove the entity that causes the problem if you do not have the necessary software or knowledge about that software to pull this off.
I guess its also worth mentioning that this is not a mistake from TGC but more of the custom content creator.
-Wolf