lads, i got the ULTIMATE solution ^^

1st: create your level, the car etc. as usual.
2nd: add a hollow cube, name it "skyblock".
3rd: apply a full black texture named "background.tga" (!)
4th: set the flags for all six blocks to "sky" (!)
5th: use the "sky.wdl" that comes with the tut pack (mod it/write your own, as you like)
6th: compile and run. and...... there comes the skybox WITHOUT any error ^^ YAY

proof: