Lol, i solved the problem. It was just the length of the file name of the .nds files
So i shortened the filename, which where to long.
This was the longest File which works: "5588 - Pokemon - Weisse Edition (Germany) (NDSi Enhanced) .nds"
The next longer wasn't working anym: "z122 - Pokemon - Weisse Edition 2 (Germany) (NDSi Enhanced) .nds"
So yeah, that was the problem, for everyone who maybe have the same problem (shortened before use the forwarder).