I searched it already, but I didn't find anything.

I added the Hoth sky to my map but it looks like this.
How can I fix that

You found the Hoth sky where and what did you do exactly? We need details.

I copied all hoth sky files from the Hoth world1 folder.
Than I pasted the sky files in my ABC world 1 file
And of course I have changed the ABC.sky file.

Are you sure you have all the files? At first I thought that was a screen shot of a nighttime sky with stars but it's actually a texture-less dome. Is "sky_hoth.tga" in your world1 folder? If it is, try resaving it and then remunge.

Most sky domes don't have bottoms, they rely on the terrain to cover that.